YANBU!! Firstly you are living with someone with bipolar disorder - been there, didn't work, never ever going back, its a very difficult life (and that's an understatement as well.)
Secondly his mother doesn't know - so you are effectively bearing the emotional burden of his illness alone as well as the financial.
Thirdly, even if his mother doesn't know about his illness I don't understand her insistence on your paying. Yes, you are the bread-winner currently, but isn't she interested in the fact that her son is off work? Hasn't she questioned the reasons why? And with this in mind, considering it was her son that caused the damage, she should be sending him the bill with an "you can pay when you are back at work...." clause not having a pop at you about it.
Fourthly, WTF? This is not normal behavior, and even if she doesn't know about his illness, she should surely be concerned that her adult son doesn't know how to behave correctly, and worse, behaved like an arsehole. Why isn't that her main concern, rather than putting you in this awful position?
Really feel for you, but like many others here (including yourself I think OP) I feel you are being taken for a ride. Yes you are supporting him financially, but that is a big enough burden alone without him swanning around thinking he can do what the fuck he likes and you will sort out the consequences.
I agree with wastedwaist that you could talk to MIL, discuss the fact that his behavior is out of order and pass the ball back to her to discuss with her son. Perhaps she doesn't want to face the fact that her son behaves so badly, but you are not responsible for him, not in that way at least. Surely she has some idea that he has a problem? Has she never suspected anything? Perhaps if you talk to her it might benefit you more long term anyway - you don't have to talk in terms of mental illness, but you could say he is challenging at times - I bet she already knows this. Good luck to you.