A bunch of posters seem to think this is some kind of personal choice the addict can break free from anytime he wants to. That just... isn't the case. Addiction is a disease. As terrifying as it is for those watching it destroy a life from the outside, that's nothing compared to the terror felt by the person who's trapped inside the addiction.
No doubt there's some part of your friend that's terrified, too, but that part of him--the free part, the human part--is no longer in control. Worst of all, what a lot of posters have said is true: you can't save him. In the end, the only person who can fight off the addiction and reassert his freedom is him.
There are things you can do for him, though, and the fact that you're posting on Slashdot (of all places!) is all the evidence I need that your concern is sincere and deep. You could abandon him as some posters suggest (he *is* a legal adult), but that isolated, coldly individualist approach strikes me as inhuman. Part of being human is friendship, and part of friendship is supporting the friend when he's at his lowest--even if he doesn't want the support. You could cut him off, and that might even be the "smart" thing to do... but to do so would thwart something in you that runs deeper than logic.
First and most important: get in contact with everybody who's really important in his life: parents, siblings, close mentors, maybe his pastor (depending on the pastor). There's still not a lot you can do, even in numbers, but it helps. Cooperate, keep each other posted, and make sure everyone close to him is making their concerns known in a non-confrontational way. Ultimatums are not helpful, and will probably just hurt everyone involved - the addict can not freely *choose* to leave the game, and an ultimatum usually only destroys other support systems. In the end, HE has to name this addiction for what it is, and trying to force him to accept it by logical argument or appeals to his responsibilities won't help... but simply knowing that everyone in his life is ready to support him as soon as he admits his problem might make it easier for him to admit to the addiction. Addicts are often terrified of what people will think of them if they come out as addicts. If your friend knows the people closest to him already think of him as an addict, then he has nothing to lose. It would be nice to let his professors know, too. It won't change the grades they have to give him, but (speaking as the son of two professors), professors are always much more sympathetic towards students if they have an *explanation* for why their coursework suddenly imploded. And, sometimes, having a professor in your corner can be a big help when it comes time to put your life back together post-addiction.
In the meantime, if you've made your concerns known, there's not a whole lot more you can do. There are a few suggestions in here I like - teaching him to cheat, thereby robbing him of the "joy" of tedious grinding, was clever - but don't take away his computer or block his ports or grief him or hack his accounts. Your enemy isn't the *game* but the *addiction,* and if the addiction doesn't go away you solve nothing. Even if you destroy every copy of Burning Sea in the world, he'll just switch to another MMO. In the meantime, if he hasn't admitted to the addiction yet, he will interpret your forced intervention as a semi-violent assault on him. He'll gain nothing, but he'll lose his trust in one of the people he needs to trust now more than ever--his friends and family. So let me say it again: don't violate his trust, however perverse his "trust" might be right now. Just let him know that, whatever happens, you ARE his friend and you WILL be there for him whenever he decides to seek help, and, in the meantime, you'll stay out of his way. Make sure he knows that he can trust you to be there for him when he admits the addiction without directly accusing him of addiction or attacking his behavior. That's a difficult line to walk, I know, but there it is.
Third, I Am Not A Psychologist. I'm getting most of this third-hand from all my many alcoholic relatives (Irish family), from a few family members in A.A., and from having seen one of my friends self-destruct on WoW during his first year at college. Get a professional's advice on this. I'm serious. Five minutes with somebody with experience in addiction counselling is worth all the slashdot commentary here combined. (No offense to slashdot.) Your college undoubtedly has such counsellors. They may be more used to dealing with alcohol and substance abuse problems, but they should still be able to acknowledge an MMO addiction and help in similar ways. If they *don't* take an MMO addiction seriously, then keep searching until you find someone who does!
I'm sorry I can't be more encouraging. Your friend is in real trouble. You can only do so much to help him, and to do too much could actually put him at greater risk. That's frustrating and terrifying (I've been there). But your investments of time, energy, patience, restraint, and (ultimately) love will pay rich dividends if he is able to recover before destroying himself completely. There's a part of him that hates your concern and your love, but there's another part, a part you can't see, that thirsts for nothing else. One day, if you and he are lucky, you might get to see that part of your friend again. Good luck, and (if you are a believing sort) God bless.