I used a 256MB RAM, Pentium 4 machine till the beginning of last year. I could run Avast with no problems, but still I wasn't still satisfied about the performance. Because every MB mattered.
So I ditched my antivirus, and started to use
Sandboxie. That way you are still protected. You can mostly install games in Sandboxie, scan the files using an on-demand scanner like
MBAM and simply copy the files to the drive you want.
My best advice is for you to try Linux, particularly
Puppy Linux, because it will run at blazing speed with your configuration.
LUbuntu has more features and is light too, but it won't have the same performance as Puppy Linux on 256MB, but at 512MB RAM it's great.
So ask him to keep his system on dual boot: for browsing, connecting flash drives and most tasks ask him to use Linux, and for playing games use Windows with Sandboxie.