^ Find a proper dose of methadone first of all. I would do this first by waiting until he was in withdrawal and give him 20-30mg of methadone. Methadone is a very potent opiate and is not to be understimated. Hell, it caused two overdoses for me and put me in a coma despite having a tolerance of 100mg a day. But it was solely because I told myself, "that can't happen to me." It can happen and will, especially when you combine multiple CNS depressants so make sure he doesn't take any benzos or drink.
To find a proper dose, take around 20-30mg since he has a fairly large tolerance, but no one can tell you for sure how much he will need because no one knows how pure the dope is and how he will react to methadone. Dose the methadone then wait 2-3 hours and redose. If he feels extremely sick still then take another 20mg, but if he feels a little better then tell him to take another 10mg. Repeat this as many times as possible and always wait several hours before redosing. That is how methadone kills more people than heroin.
The proper dose should get rid of cravings for him, but try and keep the dose as low as possible, but also make sure that he is comfortable and not high. Methadone is a horrible drug to get high on and he shouldn't want to if he truly wants to quit. After the proper dose is found, gradually reduce the dose over a long period of time. This may be several weeks to several months. Reduce 5-10mg every week or maybe even less if he can get by with doing that.
I assume that he is buying off the street and I understand why some people choose to do that, but it may be a good idea for your friend to look into a methadone clinic detox. That way he will be monitored properly instead of self medicating. Plus he wouldn't have to worry about overdosing as much as using illicit methadone and would avoid going to jail. Methadone clinics can range anywhere from hundreds of dollars to 50 dollars. So if your friend lives near one, then I suggest him going there to taper. It could end up being a lot cheaper and so much safer. In fact, I would go that method if I was shooting up that much tar.
If you need help finding a methadone clinic in your area, I will be happy to help you and your friend. I want people to get off that shit. I know how it is and I just recently relapsed. I have gone through methadone withdrawals twice, each time was cold turkey and it was horrible. The withdrawals lasted 2 weeks physically and I didn't feel anywhere psychological stable until 6 months. Your friend can handle it though if he tapers properly.
In short, tell your friend to go to a clinic where it will be legal, more than likely cheaper, safer, and easier on him. That's my harm reduction answer.
