Need to know what he has first -- what does he have in the way of soundcard? If it only has digital out, then he's going to need a receiver/speaker package. If it has multichannel analog outs, well...then he's still going to need a receiver/speaker package. Just get the best you can get for $500. Something like two pairs of the Insignia NS-B2111 and a cheap receiver and a cheap sub (this last will be the hardest part), set it in phantom center channel mode. Or I think CostCo sells something like the Philips MCD709 with surround channels, but that was a couple years ago, I'm not sure they sell them any more.
Really those receiver/speaker packages are your best bet, the hardest part will be making sure you have connectivity, because none of them come with USB.
EDIT: Yeah, like the Promedia system that Grawk linked to.