Good and serious question OP. Although this is all good advice and you should consider it heavily, I highly doubt anyone who has never been where you friend is now has any concept of what it takes to get over it.
That being said, about 2 years back two friends and I discovered methadone and oxycontin. Like that we were doing them every day and before I knew it I had been doing methadone for a year. Everyone knew something was up, they just never would have guessed what. The day I realized something was terribly wrong was when I got up off the couch after a 10hr marathon of Gran Tourismo to go outside for a few and stretch a bit. The sun was coming up and since I hadn't really slept more or less for days I was totally exhausted. Still high as hell of the wafer of methadone I had eaten a few hours back I decided sicne I hadn't eaten for days eather I would give it shot. Two bites of food and I violently vomited all over the place. I looked around at what I had become and told my friends I had to go home. I left, went home and tried to sleep.
I knew at that point I was quitting. It sucked. I felt like I had a very bad cold / flu. It only lasted for a few days, but the physical discomfort like back pain, trouble sleeping, mood swings, those lasted for a few weeks. The hardest part by far though was the addiction of DOING the drugs, not the drugs themselves. It's hard not to seek out and take them. You miss the whole process.
The best thing is to just stop.. not matter how hard it may be. Stay away from people who are doing it. I almost broke relations with those two guys, my two great friends from early childhood. You cannot be around people who do what you are trying to quit and hope to have a chance.
Since then one friend is dead from getting jacked up on pills and drinking.. that was ok.. it's when he thought it would be good to drive that he plowed into a tree and died on site. The other friend, he's going to be in jail soon. He has more tickets than one can count, drug charges, DUIs, etc. He decided to keep hanging out with the same people and kept doing the same things. He has a record a mile long already and he's only 21.
Me.. I'm doing fine. I have a good job and am currently attending college. I have tried my best to avoid drugs at all cost due to my addictive personality. I drink pretty often now, but who doesn't in college. I've done H once since then and it was so amazing I remember why I got on it in the first place. Next day the guy I did it w/asks me if I want to buy any realll cheap. No I said. If I do, I will not stop. Gotta keep that in mind and not call him back for a while. It will own you if you go back.
Long rant I know, but maybe some good advice he can use. Get away from the people that do it and the area that it incubates. Just stop cold turkey if you can, and promise never to go back no matter what.
Final side note, both of those people were relatively unintelligent and from a poor family background. Their own parents had a history of drug abuse and alcoholism. I was from a more well off family and they were always clean and healthy. Perhaps genetics provided me a better chance to have the will to stop and stay away. Some people like my two friends never had a chance IMO. I love them to death but nothing I have done has helped and they always go back, it is who they are.
best of luck, PM me if you need more advice!