A friend in need is a friend indeed
This phrase is interesting because there are various interpretations of its meaning.
There is some debate about the meaning of this expression. Firstly, is it 'a friend in need is a friend indeed' or 'a friend in need is a friend in deed'? Secondly, is it 'a friend (when you are) in need' or 'a friend (who is) in need'? If the former, then the phrase means: 'someone who helps you when you are in need is a true friend'. If the latter, it is 'someone who needs your help becomes especially friendly in order to obtain it' (emphasis is mine.)
fly by nightfly by night
(idiomatic, derogatory) A person or business that appears and disappears rapidly, or gives an impression of transience.
A slang term used to describe someone or more than one person (can be as large as a business or any other entity) who is extremely unreliable, unless they want something from you. To put these dirtbags in their place, one must confront them directly face to face. This term is different than a flake, as a fly by night DOES contact a person, but only when they want something, whereas a flake or flaky person DOES NOT.