I would not know what to do in this situation.
Do you just shut down and walk away and find a different conversation to join?
Do you humor them?
Do you ruin your reputation and friendship by telling the man's wife "You're stupid"?
Do you try to politely explain something to someone who is dumb as a crystal and will probably never be able to understand?
Do you call 911 because they might be having a stroke or psychotic episode?