Two men may use the word "buddy", but can a man call a woman "buddy"? If not, what is the equivalent for women?
The term "buddy" is not masculine, because the term itself has been used for men and women. There are similar nouns for "buddy".
I don't know whether most people consider buddy an Americanism here in London but most men my age or younger will commonly use it in place of man or mate, myself included.
In any case, connotations or dictionary definitions be damned - I will use buddy, mate, man and dude when speaking to all of my friends, male and female alike. Not because I am a feminist or anything, I simply feel those words express my character - and that matters more to me than concern with the appropriateness of the context.
Naturally though, I wouldn't speak to polite company so colloquially.
That's not very common. Something similar are terms of endearment like "baby", "sweetheart", but you'd want to be careful to avoid those if it's not your intent. They're much more intimate. And than can also be offensive depending on the person and the circumstance.
You could say something like "sis" for sister or "girl". Or even better, borrowing from Spanish, "Chica".
"What's happening Chica?" "Hey girl!"
Between two people that are dating, it's a little easier. People often come up with fun pet names. But between two friends, it's not very common.
If you're just looking for a term, a reference, you could call someone your "pal". That is also a word more common for males, but consider the term pen-pal; it can be gender-neutral. However, "pal" is rarely used in any positive sense today. Alternatively you could use "BFF" (best-friend-forever) or "best friend".
Ultimately however I think your best choice is to use the gender-neutral "friend". The word "buddy" is a little archaic and was used in a time when males wouldn't have commonly referenced to females in the same way.
If that is too general or formal, you'll need to provide a cultural context; buddy is specific to some groups, and not used in others. I would call male and female friends 'friends', and specify 'female' if I needed to. Usually, it is immaterial, so why be specific?
I prefer not to make gender the centre of every interaction; I would go as far as avoiding the female diminutive names for professions like 'actress' where it is optional, because it accepts and reinforces an unnecessary social division.
On 'buddy', in the UK this is considered an American term, and is used for example at universities or clubs each new student gets assigned an existing member as a 'buddy'. Here it is used for any combination of genders.
Buddy is not necessarily male, the word has no gender connotations of its own in present day usage.
If feminine gender is to be expressly stated, then girl-pal/ gal-pal might work.
You can have a female friend, or many female friends. But the word, friend, is perfectly neutral. Be careful with the expression, girlfriend, which is usually used to mean two people who are involved in a romantic relationship.
My experience in my American dialect is that there are a paucity of good friendship words that sound right when applied to women. I think "Guys" is becoming more and more gender neutral, at least in my personal experience, but I wouldn't say that "buddy" is there yet.
"Girlfriends" can be used, but obviously that's ambiguous, and the default expectation, if no extra context is added, would be that a single girlfriend is a romantic partner, not just a friend. But if a woman said "I'm going to the movies with my girlfriends", that would read as being a group of friends, not a polyamorous group. A man would not call his female friends "girlfriends".