yea, you do know that political correctness can also be called BEING POLITE... two of my friends are brothers, there mother in real life is paranoid schizophrenic... you know what they HATE seeing, people making jokes about paranoids... you know what they wont do though, tell random people (not even all our friends) about the deeply held problem.
One of them walked out of a Vampire game because of how someone was playing paranoid, and the story teller to this day doesn't know why they kid left game mad...
Gonna be totally honest with you, that's their problem and nobody else needs to or should even feel the need to change their behavior so that these random two people can feel better.
Of course don't make fun of them in their face, like what that story teller did (but it's not even his fault because how was he supposed to know about them if they didn't tell him?) but when youre alone with some friends, and everyone's cool with impersonating disabilities, then it's fine.
Also, one other thing, what they are doing is IMPERSONATING. very different from MOCKING. nobody is going "haha downies are stupid haha get a real job idiot", what they are doing is IMPERSONATING real life mental illnesses that would REALISTICALLY affect their characters.
Political correctness is not at all being polite, it's got the title political for a reason. Because politicians speak to the whole population when they give speeches and such, therefore they need to accommodate any illnesses that the WHOLE POPULATION might have. There is no need whatsoever for political correctness when speaking with people who you know don't need that kind of sensitive treatment.
About your friends, I suggest you tell them, if you actually care about their feelings, to open up about their situation, because it's not going to go away by treating it like tiananmen square, tehy need to tell people "so yea my mom is paranoid" and whatnot so that people KNOW not to joke about it around them. It's their life and they can either live it being constantly offended and put down, or they can look at it realistically and know that people are going to continue to imitate paranoid people around them and unless they speak up nothing is going to change.