Cortex Command (probably?)
are you trying to turn his computer into an active volcano
cortex command can get scary laggy really quickly if your cpu is closely related and/or identical to a potato
as for easy to run games:
cave story (fun fact you can even play cave story on a phone)
any goldsrc and source game, they are incredibly optimized (except for all of the latest ones, it seems like it's only gone downhill in that aspect)
any roguelike should be bearable to playit can also be quite enjoyable (maybe in a guilty kind of way) to play rpgmaker gamesokay i actually dont have a lot of free games that aren't random junk so i'll just throw a couple more links i guess
ijihyper princess pitchhero corean untitled story (direct download link)
marble blast gold/platinum/etcstabyourself.net has a couple of cool gamesknytt stories is absolutely GREAT and has tons of really really good
custom maps! (
knytt stories+ and
knytt stories EX if you want to get deeper into the whole custom level business)
within a deep forest is also really really good