Just for clarity's sake, this is the Apple version of the card? I would assume so since you say he has the 24" apple display. However, he may be using a flashed card and a DVI to displayport adaptor....
If it didn't work in OSX it would kernel panic or just not display anything. My guess is that A) something is messed up with the card, in which case he should set up an exchange with apple. or B)He does not actually have it hooked up correctly, even though he says he does. I'm not saying he's incompetent, but there have been many times in my life where I swear I've got something set up correctly and I don't.
My suggestion is for you to read the directions and then check if you think it is set up correctly. If you do, then I'd call apple and tell them your buddy received a defective card.