Would he consider therapy? He needs to learn that what he grew up with isn't acceptable now that he has control of is own life. It never was acceptable, but as an adult with his own home he no longer needs to surround himself with that dysfunction. He's letting the patterns of his childhood continue their destructive ways, and it's going to tear the two of you apart unless he takes action on it.

He needs to be told this directly, so he understands that this situation has long term consequences. You know that you're not going to be able to live with her actions happening even one time more. The only way that he's going to be able to fix the relationship is by accepting the truth and changing what he can change, which is not his mother. The only thing he can change is himself, and having help (therapy) and support to get him through that is the best way to get it done.

If he can't protect his home and his partner from his mother peeing all over everything like an untrained puppy (but worse because she KNOWS she does this), how can he protect you from other things that happen in life? And again, he doesn't need to turn his home into a reproduction of his childhood.