Trying to help 91 yo dementia patient deal with dying wife. Any advice?

I guess it depends on a few things. How advanced is the dementia? Does he "know" his wife is dying? Can he understand this? Where does he live? Home with her still?

I think there is no way to get a demented person to "deal with" much of anything. Their minds don't work in a normal way. They will "deal with it" in the way their own mind works, and that is the guide that you will have to take, as there really isn't a choice and this isn't something that can be fixed.