I think once you step into the territory of trying to persuade someone, you're trying to control that person. If he asked for your opinion, you can state that it's a terrible idea for [reasons], but he is free to take it or leave it. Think of a greedy merchant in a bazaar that's constantly trying to pitch things, grabbing customers by the ear and trying to hypnotize them. Compare that to a till worker at a fast food restaurant. The person at the counter really doesn't care if you come asking for food or not, but they do their utmost to give what is asked (assuming they're competent/considerate). Opposed to this is the merchant who is heavily invested in getting people to buy his product. Instead of selling an item, you're sharing your view of reality. Respecting free will and having humility is the role of the till worker. The merchant is one who wishes at root to control another person (and their view of reality).
I think it's a waste of time. People need to want to change. And I mean viscerally want. Often, changing course only comes after a lot of suffering. I don't think an exposition on how wrongheaded he is will change his mind. He's enjoying himself, so his system one will have no trouble compelling system two to come up with justifications and explanations.
He doesn't need fixing as far as he's concerned. I think you should save your help for someone who is actually asking for it.