What do I do about a friend who is setting himself up for romantic failure? Tell him it's not a good idea or do I let it go and hope it turns out to be okay?
It depends. Is he looking to just keep hooking up? Or an actual relationship?
You don't even have to straight up say its bad. Just tell him there may be some things he should be concerned about if he wants this long term. I would give advice, but now necessarily tell him what to do.