Yeah, I think the key here is that he could play before and now he cant with the same system. Thus, it really desnt matter what his hardware is. Even if the settings were changed to be higher than his hardware can handle it would have a detailed error message or he would get about 1 fps.
Killer,
I dont know what has happened in the last period of time on that computer but it is possible that somehow your game files (or just one) has been corupted. Probably the easiest thing to try is to reinstall HL2 (since that is what gives the message). When you uninstall it you should be able to save your settings so as not to lose any progress made, and then reinstall the game files and update it fully via steam. See if that workes and report back sometime.