It's a horrible road but the only thing I know of is calling the cops whenever he gets physical, causes her to run out of the house like this, etc., etc. Enough reports of domestic violence & you can get him removed from the decision making process. The you (& your brother, hopefully) can get her evaluated to see what kind of help she need. Otherwise, this continues until he harms her horribly or she is lost & alone wandering around.
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@Rabbit2047, I have no sympathy or anything for abusers so personally I'd call the cops if he even raised his damn voice. Get him out of the picture & take care of her. People with dementia can't help their condition. Abusers can, but don't want to. But that's just me.
I'm sorry, man. It's a real 2 dead ends but you still got to choose situation,