I'm going to go on a limb and say they met via internet. Am I right in guessing that? I don't think there's anything in particular wrong with dating someone online (I am myself), but there's risks....

At any rate, a little more info on how they met might help.

Part of it can be (provided on how things are done between the couple. My girlfriend and I sought to eliminate this factor ASAP) falling in love with what you think someone is rather than who they are. If he's doing that, I'm not sure how much you CAN do to effectively break them apart. There's a powerful sort of assertion that comes with that, and it'll simply boil down into a matter of (to take his perspective) "I'm right, you're wrong. Done."

I would think that the best thing to to is sit your friend down and explain your concern. Don't try to attack her or their "relationship." That won't get you anywhere, as he'll shut down and go on the defensive. But explain that you are concerned about the effect this girl has over him. Explain that you're scared of losing him, etc. Things like that. I don't suspect that'll make him instantly snap too, but it'll bring some perspective to him.

I hope I was of some help. I hope things work out for you and him.