Have you been able to ask the family directly about this? My experience with diagnosed dementia patients is pretty limited, but I am not sure that it's common to withhold that info from a person. The two diagnosed people who come to mind both thought that the problem was with the doctors, not themselves.

Is it possible your friend has never been diagnosed? The test involves being asked a lot of questions that can seem pretty silly, so it's sort of memorable.

I do think that when you live with someone, it can sort of creep up on you. You just think that they are getting more forgetful because they are getting older. My neighbor would not have been diagnosed at all if he had not had a medical emergency because he forgot he had taken his medication already. It was obvious that something was wrong, but it was subtle.

I hope you find out what is going on with your friend and that he is getting the help he needs.