It definitely sounds like a brain disorder. I would guess dementia. It doesn't always manifest as amnesia, or not at first. It can attack other parts of the brain. Perhaps it's causing hallucinations.

I feel very sorry for them both. I hope he gets the help he needs. I doubt he's being malicious, but I also feel that your mother feels attacked and betrayed and it's not her fault to feel that way either. It's very understandable.