Now seems like a perfect time to upgrade to Win7 though..
Assuming his XP pc can handle it
Id go with that though, download it using a torrent, and use your own key. Just make sure you have a decent anti-virus running, and do a scan of the download before using it.
Though I must also admit, im fond of NT5 (2000 and XP), and gradually grew to like Vista, but 7, whilst my experience with it was initially fantastic, im somewhat dubious about its longer-term reliability - it seems to have awful problems with memory leakage (which never seem to affect my XP or Vista systems), which is surprising as Microsoft have been working to address this for many years.
Basically, im still undecided on 7, though for the majority of the time it behaves, and boot up (from cold) times are excellent on the odd occasion I have to restart (otherwise all my systems only get hibernated for months at a time so boot times are irrelevant, though hibernte times are far far faster in 7 than in Vista lol).