I was addicted to EverQuest throughout all of high school. I pretty much missed that _entire_ time of social development. I wanted to avoid it, and EQ made it easy. EQ was the kind of game where you could sit there and do your homework while your group was meditating. I still kept my attendance up and had good grades. I just avoided social interaction.
I had parents and friends tell me that it was a bad idea, but I just turned on my defense mechanisms. It was Them Versus Me. By taking that approach, they became the bad guys. They became someone who doesn't understand because they didn't understand.
The only person who *might* be able to get through to your friend in a direct way is former MMO addict. But of course, as it's been reiterated on here plenty of times, the only one who can truly get through is the addict himself.
In my situation, it was a mixture of things that got me over my addiction. Firstly, the game started to get old. I tried other games like Asheron's Call, Anarchy Online, Dark Age of Camelot, SWG, etc. These continued my addiction for a while longer. But eventually you find the same things in every game and they just keep getting more boring.
Secondly, I started to really re-evaluate myself and what I was doing with my life. I knew I was a social misfit and that had to be fixed. When I started college, I quit my games and started on a clean slate. I developed for a while and it was going great. I was making some real progress. I even got a girlfriend that my friends gawked at.
Then EverQuest 2 came out. Yeah. I went back. It was a relapse, you could say. I went back to my habits of nothing but gaming. I still kept up with my school work but I avoided my girlfriend. Eventually, she gave me the ultimatum of her or the game. I tried to convince her that it was only temporary and that I'd be over it soon, but she wouldn't listen to that. So, I chose the game. I told myself 'I don't want a girlfriend who would give me an ultimatum like that. I want someone more patient.' In retrospect, I know it sounds pretty stupid. This is just how I reassured myself as I continued to play EQ2. It's just how the defense mechanisms work.
I regretted that decision for a long while, though. Eventually, as I expected, I got bored of EQ2. And WoW. And all the other games that were popular around 2004. Eventually *none* of them sparked my interest. I graduated from college and started a job. By that time, I was totally done with MMOs and I haven't looked back.
So, to sum up: it sometimes unfortunately takes big life transitions like going from college to a career to really wake someone up. Right now, in their life, they are at a point where they are content. They enjoy their games. You cannot argue this with them because it will only make it worse. You can only try to be subtle about it. Try to make them jealous. Bring over hot girls that they can't have. Go on trips with your buddies and come back and tell him how awesome it was. He won't be receptive at first--not at all. He will have some clever retort. But you will have planted the seed of corruption. Eventually, when he's tossing and turning trying to sleep, these things will eat at him.... not that I speak from experience in this department =x
Eventually, he'll get over it. Fortunately, MMO addictions are a lot easier to recover from than drug addictions. They can ruin lives just as well, but fortunately once you get a taste of some good fun in real life, you quickly forget why the hell you were playing those games in the first place.