"Well, addiction to marijuana is the most common types of addiction now days" Erroneous on all accounts.
Back to main topic though, Relyt has some good ideas. It's important that your friend does talk to all his friends who smoke. This is probably THE most important because your friend still smokes probably because he sees his friends doin it and can immediately regain the 'why not' mentality for smoking. If the friends don't wanna help in his quitting, he needs to detachment himself from that group.
ANOTHER IMPORTANT NOTE- DO NOT TALK TO THE DEALER ABOUT QUITTING. Simply stop contacting him. You do not want to get in between a drug dealer and his business. I'm not saying bad stuff's gonna happen or even that he'll understand. What I'm saying is that it's unknown so it's best just to cut contact from the dealer. He probably will just continue on doing his thing for other people anyways.
Finding a hobby is good too, basically anything that will SUCCESSFULLY fill in the spot where MJ once was will do.
In the end, it's all up to your friend whether he will quit it or not. Just like someone mentioned above MJ is addicting just like video games are to some people. It's all psychological. Your friend needs to realize what he's doing isn't what he wants to do and is detrimental to what he really wants to pursue. It's gonna be how he perceives smoking. Because for example, I smoke more than your friend, but I don't find it affecting anything I'm trying to pursue in my social/academic endeavors.