Star Trek Voyager: Elite Force is a very solid shooter, and old enough that it shouldn't have any problems with his hardware. I replayed it a while ago, and it ages well. The story is passable, but not intrusive (especially if he tolerate Metal Gear's cutscenes!) so that shouldn't be a problem. I'm not sure if the gameplay could be called varied, but the level design is very good, so despite the face that I have a low attention span, I was never bored. So that's my first recommendation.
And if he liked Half-Life 1, I'll assume he's played the sequel? If not, that's an obvious choice.
Serious Sam: The First Encounter has no story worth mentioning, and should run without a hitch, so he could try that. Far Cry is cool too. So are Medal of Honor: Allied Assault and Call of Duty II.
All shooters, I know, but those were all I could think of within those criteria.
Edit: Also, both Max Payne games. I personally love the atmosphere and story, but they are easy to ignore if you don't care. Also, Bulletime = funtime.
Edit 2: I think that Metal Gear Solid 2 actually exists for the PC as well, so if he hasn't tried that either, go with it.