My husband keeps telling me things that his Mother is telling him, She died when he was seventeen. I just go along with it and say, "Oh, that's nice love", or something else suitable.
He asks me "Where's Holly?", (his dog who we had put to sleep about 4 yrs ago), I tell him she's ok, she's sleeping. (Well I suppose you could call it that)
My Mum had Dementia and was always telling me she'd seen her Mum or her Sister, both long gone, but never my Dad,(he died in 1992), who she adored. She never mentioned him. I wonder if she blanked that out deliberately as it was too painful.