I feel so bad for my dad. He handles mom's dementia well, going to extremes to please her. But now she hallucinates and thinks he's causing it. She thinks that he appears and disappears, and invites strangers in the house. When she questions or confronts him, no answer he has is good enough. She's blaming him constantly.
Is dad taking care of mom alone? Do they have any assistance with housekeeping, caregiving?
I only tell her these are not real if she is upset by them. I explain that her prior stroke is sending mixed memories and illusions and to talk to me about them if she is afraid.
I can only imagine the fear of those hallucinations for the person experiencing them - esp when they know they are straddling reality & other.
Anyway.... if your mother normally trusts your father, and they have talked before about what is happening to her, could he try drawing her a diagram of what's happened to the bit of the brain that controls vision? She might find it reassuring to know that it's her visual centres that have gone wonky more than that she's imagining things exactly.