I'd tie him down and try to slap some sense into him - but since that rarely works, I'd suggest this, and it'd help if you have a sympathetic mate: sit him down somewhere quiet (local pubs are always good for this), buy a round and explain things. (wait until he's partway through his drink before you start - chat about something else until then) Tell him the facts as you see them, and be as impersonal as you can about it - insulting her won't work, no matter how deserving. Explain that: - since she both dumped him *and* got a new boyfriend, it looks like she wants nothing more to do with him - even if she was still single, he wouldn't be doing himself any favours trying to chase after her. - if she might have cheated behind his back once, who's to say that she won't do it again? - he's not the only guy who's had a rough break up, get over it, there's others out there, etc, etc. (actually, if he's still moping by this point, some slapping might help. who knows?) Seriously, the sooner he get's the idea out of his head, the better. I'll admit, the end of a three year relationship is going to be hard to get over, and it's not something you'll see sorted overnight. If you can convince him to drop it, though, that's half the battle over.

You can't make him, you can just offer advice. He does not sound very interested in the facts because he must be still hung up on her. He has to come to the realization that it is not going to happen on his own.