Both Starcraft games should still work on OS X, SC2 might have performance issues especially with no GPU, so your friend may have to put up with Broodwar instead (good luck!). Both games are free to play nowadays (though the official Broodwar ladder requires buying the Remastered edition, or alternatively using a third party ladder).
0 AD should work on OS X otherwise (it's RTS in the vein of Age of Empires), and if he's not looking for an RTS, there's always Battle for Wesnoth, Freeciv, OpenTTD and co depending on what kind of game he likes.