You just drop it. If someone has dementia, they may give an incorrect answer without realizing it, or they may forget incidents that happened or details about them. They either truly don't realize the error, or they suspect a mistake but feel too proud (or scared) to own up to it. Correcting or challenging them adds to their confusion or frustration (or heats up their denials of having misspoke). But it does little to solve anything. Better to let the matter drop.

If the person hasn't had an actual diagnosis, it's important to get a checkup so a doctor can verify and try to pinpoint the cause of the dementia. That will speed appropriate therapy, such as medications that can slow early memory loss in people with Alzheimer's.