How can I help my friend?

I have a friend who is a real delinquent. His favorite thing to do is go and start fights. He's been to jail many times for it. But, he met this girl that he really likes and she's good for him and keeps him in line. His delinquency was pretty much gone after he met her. He was going to tell the girl that he really liked her, when she came out to him that she loves someone else. Now, he's completely shoved her out of his life and is back to his old ways. Just yesterday he started a fight and left the guy a bloody mess on the pavement. What can I do to help him again? I still like him a lot as a friend and I want the best for him.