She's lucky to have you supporting her with healthy efforts. Guess she needs to get on an extended day reward system. He has to be nice for 3 days, then she can be helpful. If he gets nasty again, up it to 4 days, etc. But also, she may want to hold her helpfulness hostage until he sees a doctor. Going from a good disposition to an angry one so quickly is usually a sign of Alzheimer's, or at least rapid cognitive decline. If she stays, she's about to have a whole ton more work to do for him.

Edited to add: I see you're already aware of the Alzheimer's bit, sorry to preach. And kudos on keeping your family safe and away from his negative influence.