My dad has Alzheimers. He doesn't know for sure if he has to have a bowl movement and he can't go by himself . At least 10 times a day he says he has to go but only passes gas. Even when my mom (who has to help him remember how to get on the toilet and wipe his butt) tells him he's already gone to the rest room, he insists on going approx 10 x's a day!! When he's not on the toilet he's talking about it (or tries to talk about it). Is this normal for a person with Alzheimers?
Beyond that, a lot of older people get fixated on their bowels, after all when it's not working it is often hard to think of anything else, and dementia just magnifies their obsession.
I would also discuss with his doctor. He may have some elimination issues that no one knows about. Maybe, he needs to alter his diet with more fiber so he has more complete bowel movements. I'd ask about that.
Attending to his toileting needs sounds pretty exhaustive. It may be that your mom needs some help in the home with that. That's a huge amount of work for one person, especially, for seniors.