You have to sit down with him first. Tell him why you are concerned and point out what those pills are doing to his kidneys, his body, and his friendships. It's straining all of these things. Odds are, he'll tell you to shut up, tell you he'll try and quit, or he'll agree and will seek help with you.
If that doesn't work, offer to help him. Give up something you really like as long as he isn't taking V's. Pay him for each day he doesn't take them if that's possible and he'll do it. Try and make him see it as a challenge you are going to help him with. Make sure you talk with him and not to him. Don't lecture, but talk with him and let him say his side.
Then, if that doesn't start working soon, you're going to have to tell his parents if they are the kind that will care. He'll be pissed and will probably stop talking to you at this point, but explain that to him from the beginning.
Pill-heads can be some rough looking people. I had a buddy that was a walking pharmacy for about a year or two. Everytime you'd see him, his eyes were in a daze, he walked around all sluggish and worried about what pills he had and what he needed to stock up on. He was slower than a year before in reacting and in conversation. Unless you really knew him, it wasn't too obvious except when he was in "party mode" which usually consisted of doubling his normal outrageous amout. He was able to get off them pretty much cold turkey. He got a new job and really worked hard for a couple months. Didn't really have the time to pop pills anymore. He does them occasionaly now, but he's quit selling and it's never more than once a week...so he's doing better now.