What happens when people who are bipolar develop dementia?

Asking for my SO, who is at least as frustrated over my MIL's nasty behavior as I am.

I'm not sure she has neurodegenerative dementia. I wasn't around for her first diagnosis of "dementia" and if it was based on objective medical tests or her not passing these doctors' office quickie questionnaires.

And I wonder if some of her awful behavior can be treated with a different kind of medicine that is not antianxiety, but targets depression. Can mania manifest as spells of active hateful behavior with confabulation? It seems like bad news and stressful situations set it off.