Some budget recommendations:
NFS: PU (edit: unintentional smiley

) is a great game, and it should be dirt-cheap by now. You can also find it in a racing bundle with Superbike 2001, NFS3, and Nascar 2000 (IIRC), probably for under $15.
I've ben playing the original Deus Ex, and it get choppy on a P3-866 and 9100! I think my CPU is the culprit, so his 1.1GHz Cel may reduce the occasional dips into the teens. Pretty good game so far. You mention DE:IW, though, so maybe your friend has already played the original.
You could also give him Wolfenstein: Enemy Territory on a CD.

Great game, but his CPU/vid card may be a tad slow for it (my 64MB 9100 begins to choke at 10x7).
Battlezone was a great game that I wouldn't have discovered had it not been bundled with my Voodoo 2--give it a spin.
If he likes RTS, check out Battle Realms, Warcraft III, and Age of Mythology. The first should be very cheap and is a pretty refreshing spin on the genre; the second and third are obviously big-budget and very well-made, story-driven RTSes. I don't use multiplayer with RTSes at all, so I'm just talking about the single player experience.