Talk to his girl. Prove her that your friend needs some guidance which, at this point, can come from her only. Cos we're talking about FEELINGS, which ain't sth you can control, nor just talk 'bout 'em and cool 'em down by doing that. This is not about a stubborn point of view- "I feel sth and I cannot ignore it". Not at all.
At this moment, when your friend cannot expose his feelings to her anymore, they're doomed to die out in a LONG period of time. And cause A HUGE AMOUNT OF PAIN. One thing that keeps the feelings "burning" are his memories. Every time he remembers the beautiful moments he has experienced with his EX, he feels pain. While feeling it, he wants her to come back, cos he was quite fine when with her, so basically he sees his EX as the only way out of the blues.
This is where somebody who can make his feel good steps in. Memories must be "overwritten" by newer, fresher experience.
Of course he's not gonna "switch the playaz" in one night, but it's gonna be WAY BETTER than just ignoring her AND ONLY THINKING no more superb moments are gonna come while being without his EX.
It's cruel, but it's true. When feeling down, YOU must HAVE somebody by your side. No matter if you're a DJ, an AFC or whatever. This is when someone emotionally "tougher" HAS NOT TO BE YOURSELF.
It's kinda messed up, but I hope you'll talk to his current girl and if she's gonna be cooperative, this'll help him for sure. Believe me.