I am a bit of a cynic, but any time someone's response to a question is, "I would never", they already have. And this solidifies my belief. You have done your duty. Be there for your friend when/if he ever figures it out.
My wife's sister was completely delude to her 1st husband, probably worst than this. She finally got a clue when she got venereal disease. Some folks just gotta learn things the hard way.