I don't play World of Warcraft for the following reasons:
1. You have to pay
2. It's not THAT fun, but I can see how it appeals to most people
3. Too many 1337 players that will own my a$$.
4. My family will think I'll become a WoW addict like my brother was, and will quickly tell me to stop playing and bug me all day etc.
Enough about me and more about your friend. What can you do? Obviously deleting his account won't help, he'll probably just make a new one and get pissed off at you. I'm not sure about this, he'll realize it later probably about the amount of time it's consuming from his life.