If a friend who's gained weight asks you if she's gotten fat, do you spare her feelings or be completely honest?

Well, it can be mean at some points

Well, it would be mean if you were pointing out a flaw in someone purely for the sake of making them feel bad, yeah. Criticism is sometimes necessary though, and certainly justified if they are opening themselves up to it.

I meant criticism is not always necessary

Yeah that's what I mean, you shouldn't criticize someone for no reason but if they ask you for your honest opinion it's okay to give it to them.