I think that it was from the Caregiver Education tape.
The man lived in a ASL and he hummed/sang all the time. He was running around the ASL, all along with a fellow resident. She was a cute lady with a smile and believed that he was her husband.
The man had sang with a man's singing group. And his real wife and daughter came to take him to a concert where he led the other singers.
The whole way there he kept asking where they were going and why. His wife and daughter kept answering him, and he kept asking.
When they get there and all the men are waiting, along with an audience. One of the singers goes to the mike and introduces him and tells everyone that the man suffers from Alzheimer's.
He goes up with the other men and sings excellently and naturally. Such a contrast from how he was shown previously in the video.

I couldn't help but thinking that OUR "useless family members" would have watched that and said, "See there isn't anything wrong with mom/dad. Look at him, sure he's a little funny, but he performed professionally."

Maybe, to deny reality is to avoid responsibility?

Just to mess with people in denial, you could ask, "What would it mean to you if (insert person here) really has dementia, and really needs help?" Or, "What do you think would be expected of you, if it's true?"