I'd just install gpuminer, although it's not a "service", you can have it auto-start mining on bootup. Using the -f option, you can even have it mine while he's playing games with minimal impact to his gaming (although mining is pretty slow).

I currently use -f90 which works great. If he complains about it slowing down his gaming too much, just increase the -f90 to 100, 110, 120, or whaterver.

This has the benefit of once it's setup, it's automagic and nothing for him to fiddle with.