I've only seen stuff that you're planning to do(Kaos) happen on T.V., movie or CSI-wise. Some ends in epic death, few, with rare success. The successes were just the main guy who wanted to help the friend, lives but either him or his friend handicapped. Or the gang involved guy lives but with a terrible and regretful life, while the friend who helped died.
If his life is already bad and he can't handle the situation with his family anymore, and also you can't convince him to bear it, let him choose the path of his life himself. Even if your friend could take a long time efing up his life, he would learn from his life long's lesson from his mistake afterward and possibly get out of the gang. That's if he's lucky.
If you still wants to be a bit involved, have a sort of last min word with him, saying that you'll always be there for him no matter what. And if he ever needs help, you'll be available.
Don't be rash about helping him though, survey the situation. If police involvement is needed, contact them. There are many laws that could help him lessen his arrest with gang involvement. Many J-V, lawyers and helpers now-a-days. (At least in the U.S.).
You could just contact the police in the first place, but with some cases, that would not work. He could be arrested without being matured first-hand by cruelties of gangs and not learn his lessons. And all ties of your friendship would be severed. Thus, you could be effing up his life faster for him.
So, again, survey the situation.
Well, I hope that will help. I'm not much of an advice giving person anyway. \: