Keep in mind she has a Mac, not a PC, so some older games might not be available on Mac at all. Its really only been in the last 5 years or so that games have been more commonly getting PC and Mac releases simultaneously, Mac gaming used to be a laughable oxymoron. I still hate Macs myself, but they at least are getting games now.
As far as story based shooters goes, you've already played Half-Life, so everything else will be downhill from there. Too many first-person shooters these days are aimed primarily at multiplayer audiences, with single-player as an afterthought, and even when they aren't, the story is usually so-so at best. We end up with the Halos and Call of Dutys and even worse, clones of them. I actually like the Halo games but not for single-player, I only play the campaign if I do so co-op. But story driven shooters are few and far between.
I'm drawing a blank on what games are available on Mac and 360, are there any other genres you like besides FPS? Any you can't stand?