Re: odd question: program to intentionally crash WoW
All you can do really, is talk to him about it. If he's computer savy, he'd catch onto such stuff, if not just renaming a file would leave him confounded. Do not try to upload something over the net, which would serve to DoS his computer, as this isn't exactly legal; and depending on how things are between the 2 of you (you seem to mention friend in past tense here), this could get nasty. In the end, people will make their own choices in life.