How can a mad scientist's friend address him? For example, when engaged in a conversation with other people (who don't know the scientist), he mentions having such a friend? I suppose a loony sounds too harsh. Is there a friendlier way to suggest he is a bit of a lunatic?
When addressing him, her or it, using their name is generally deemed acceptable, perhaps appending "sir", "ma'am", "oh great one" or a list their doctorates. After all, mad scientists tend to build death rays so politeness seems wise.
When speaking about them to other people, "My friend <name>, the mad scientist" seems appropriate. Perhaps "My friend Dr. <name>".
You seem to be implying that there is something wrong with being a mad scientist. On the contrary, your friend is likely an informed, intelligent, creative and unconventional person! They simply have different interests (nucleonics, genetic mutation, robotics, world domination) than the typical person (sports, TV, movies, drinking, politics). People vary; there is no specification or one true way to be human. Though I sometimes encounter... unpleasant people who seem to believe that whatever they are used to or approve of is the only acceptable way to be human, and that they can and should reject, oppress and/or kill anyone different.
I speak as one who greatly enjoys the "Girl Genius" online comic, with many mad scientist characters. Bwah-hah-hah-hah-hah! Tremble and obey!
Part of the problem here is cultural, rather than strictly linguistic. It's not quite possible to describe someone who is mentally ill in a completely nonoffensive manner, because the concept of not thinking clearly is itself considered highly insulting. Even a lot of native speakers struggle with this, when they have to talk about friends or loved ones who are mentally ill.
This is not to say that English doesn't have friendly terms for thinking that goes outside of established norms; it does. But they tend to carry connotations of agreement, or at least of being able to see reason in the person's thought processes. You could say that someone is a "visionary," for example, but this tends to imply that you agree with his vision. You could call someone "unconventional," but there are degrees: without further clarification, there's an implication that this person doesn't stray too far.
If you well and truly can't square your understanding of reality with someone else's, but still want to speak about this in a friendly manner, your options are somewhat limited. A common tactic is trying to minimize the degree of insanity, with terms like "eccentric" or "odd" (though this latter is a little stronger). A related strategy is to play to tropes: "He's a genius, but you know how they can get sometimes."
Another option would be use an expression of pity. Some consider this offensive in its own right, but with the options being so limited, it can still be a lesser evil, especially if you cannot minimize the issue. You can be indirect about this by saying that the person "has some problems" or "isn't well": to be more direct, you'd say that the person "needs help" or "is sick." The person being spoken of isn't likely to take any of these very well (though it's more likely with the more indirect statements than with more direct ones) but it at least gets across that you wish third parties wouldn't think too badly of him for it. That's not a great option, but it can still be preferable to more overt hostility.
I guess it comes down to this: who is this person talking to? What does he really think of the mad scientist, and why? You've touched on a delicate issue: like I said before, even a lot of native speakers have trouble expressing it well. Depending on just how insane this scientist is, depicting that struggle may be the best way to make it as inoffensive as possible: his friend can't really find the right words.
"fanatic near orbit astral body enthusiast" -- if you want to be humorously pedantic as the word stems from lunar + fanatic
However I think if you shorten that all down to 'enthusiast' and widen your eyes a bit as you say the word your audience will catch your drift.
Another tactic would be to give a feined attempt at describing what this person is enthusiastic about, and then I'm certain their enthusiasm will bubble deep within them and they'll soon be interrupting you to describe it themselves. And after a few minutes the audience should be well informed as to what kind of lunatic they are dealing with.
Remember everyone is normal until you get to know them.
For people like myself and others who identify as mad, psychiatric consumer/survivors, and crazy people in general, being called by their name, although Mr. Man or "hey guy" or similar derivatives is okay by me. Not that I speak for all or any other crazies, but I would guess that most self-identified mad people are in general agreement on this point. Hey respect for the people around you! Whodathunk?
The terms "mad" or "psychiatric consumer/survivor" are similar to terms like "queer" or "crip" - it's a "taken back" term used by the people who identify as those terms, and not to be used by people who don't identify as those terms. "Lunatic" is a profoundly sanist term (sanist: you can look it up) as are the majority of other terms others here have suggested. This is why "lunatic" is sanist, it comes from a "crazy people are bad" perspective, when most of us are pretty awesome, interesting, and nice quiet people because we're used to people being assholes to us and don't want to be villainized more than we already are.
Cheers to my cray-cray peeps.
Depends what you mean by "mad". "Mad" could mean a creative crazy type -- very lateral thinker. If you mean his personality is really out there, I like "he's got a 2-sigma personality" (with reference to a bell curve). If he is a crack-pot, my favourite is "he's got a few kangaroos loose in the top paddock".
Adjective that can mean amusingly unconventional and idiosyncratic. "He's the zaniest guy you'll ever meet."
Lately has become an informal way to describe anything non-conforming. "What a crazy guy!"
It sounds like one word won't be enough, given the requirement. So probably a qualified use of eccentric (as suggested by d'alar'cop) is best. "He's extremely eccentric, verging on crazy."
Many of the replies (excepting 'high-functioning sociopath') are entirely subjective and can mean anything. Better to know the diagnosis (if there is one) and give the appropriate medical term.
If speaking in jest I might say he was 'as mad as a March hare'. March is the breeding season for hares, in Britain, when they get up to all kinds of antics.
In a similar way it is no longer acceptable to call someone a 'cripple'.
"I have to warn you, my friend is a little odd; no I'm not; yes you are; don't really have any friends, now, do you? Shut up you freakin lunatic! why? if you have to ask, I can't tell you; you...jinx snaps! MMmmm... mmmm.. mmmmmmm...; now who's your daddy?... what's that?... I can't hear you!... cat got your tongue?... hey... where did everyone go(?)..."
Does it have to be a word with absolute meaning, or just a word that implies some quality? You could simply use the name of some well-known mad scientist public icon to refer to that person. Dr. Evil? Dexter boy genius? Professor Farnsworth would be my logical choice of self-reference in a mad scientist context. Of course, the reference has to be sufficiently well known to actually have a meaning to the target audience.
I think this way you can use a word that expresses more qualities at once and therefore is a much better fit to that actual person.
Eccentric? It depends, you could use eccentric, but is that person actually eccentric? A "mad scientists" doesn't necessarily need to behave eccentrically. The very concept of eccentricity, both technically and figuratively describing character, has more to do with being "deformed from a state of normality" but not necessarily "excelling over a state of normality" which is a more important quality of a mad scientist than the fact he is not like everyone else for being one.
Mad scientist IMO has less to do with being just being "odd" or eccentric, a mad scientist will likely also be tremendously rational, which I guess normal people might still interpret as odd, but simply because of the lack of insight into that fevered brain. A mad scientist IMO implies highly "experimentative" (sorry I like to "make up" words) and inventive person, who approaches subjects analytically, be it in a mad way, make parallels between stuff taken for "random" like human emotion and formulate it technically and seek automation and modification possibilities. Mad scientist IMO also implies at least moderate, if not developed polymathism. A mad scientist must be "self sufficient" and "vertically integrated" so he can create a doomsday machine alone or at most only using the help of someone named Igor, but preferably using obedient specialized clones of himself. Not to mention the mandatory "secret lair" and unique transportation vehicle/s. Crazy hair and trademark laughter? You bet!
Which brings us to the climax question which should have been first, had my logic not went "little endian" into my head, is that person really a "mad scientist", or more of an eccentric person who makes Arduino based gadgets. IMO, "mad scientist" is not a term to be used that lightly. The previous level I guess would be "super/hyper/uber-geek" or something like that.
Keep in mind that a "lunatic" is not even "a thing" according to the scientific mainstream and official government nomenclature. It is a deprecated term formed of a misconception which IMO is perfectly fine to use in a friendly joke context. I think that after 300 years it is safe for it to "be funny". A person who would be offended by it will likely be offended by a lot of other seemingly harmless things. More of a character trait and/or mindset really. For example, in my native language "to be (absolute) freak at something" is a modern slang for being (exceptionally) good at something, e.g. freakishly good, and about 95% of the people I communicate with on regular basis understand the reference. 5% will either fail to understand and ask why so hurtful, or act the hell out of it. So it is OK to make non-offensive words offensive, but not the other way around? What is stained with insult must remain stained forever? We should take Looney Tunes off the air then! :)