Or is it just something that has been impressioned onto him? iMovie isn't really capable of "good" editing, and there are a number of programs available for a Windows system capable of claiming just as many professional titles and uses as Final Cut Pro HD. Including: Adobe Systems' Premiere Pro (which can be had with editing bundles from Matrox), Avid Xpress Pro, Avid Media Composer (very high end), along with Sony Vegas (much less used/known).
Windows XP Professional will provide a stable application base, but you might consider getting a prebuilt system (while you may have no problem working on the computer, will he have a problem working on it?) from Dell or Apple. The Dell Precision 690 can be configured very similarly to the Mac Pro, and can ship with very attractive industrial warranty packages (the Precision 690 isn't targetted at the average home user, its a workstation, which means Dell sells industrial/commerical support contracts for it, instead of home user contracts (while the Mac Pro is targetted at the semi-professional user)).
I'd suggest at least 1000GB of online storage, and 2-4GB of RAM, the 690 can be configured with a pair of quad cores, Quadro FX graphics, preconfigured quad monitor display solutions, and I believe Dell still offers the Adobe highlighted keyboard. The other advantage to the Dell (or the Apple that Bob suggested) is the warranty, if anything goes awry with the system, it can be repaired for free in most cases. Going with a Dell will likely give him some more advantages as its a PC, most schools have an inhouse IT/helpdesk that can provide PC support, but not neccisarily always Apple support.
I'd also look at M-Audio, E-MU, and Digidesign for audio if thats something he's going to be needing. M-Audio and E-MU won't provide full professional solutions, however they'll provide a decent capture setup if he needs to rip audio into the system.
Digidesign produces pro level audio manipulation equipment, including ProTools|HD 3, I'm not sure if that level of audio mixing goes over his needs though: