I don't think $500 is enough for prebuilt. Are you willing to raise it to $700?
Another option is if you have any friends or relatives that are good with computers. Maybe you could buy all the parts for $500 and throw them $50 to put it together for you?
He dosent need to buy a whole new computer if he can tell us his specs...i have a VERY old MoBo (AGP4x) that had a tnt2 in it untill i got a GF7600GT. The only problem that im concerned with when it comes to my comp is the dog slow CPU lol. If you can post your Motherboard then we might be able to save you some money =O. It also depends what games you want to play....
If he won't build one then it's unlikely that he'll be comfortable doing major system upgrades.
If $500 is your cap, you may want to try eBay, craigslist, the local classifieds, etc. I certainly wouldn't call it an ideal route, but you can find some decent deals on systems that will suit your gaming needs. If you are unsure on what's "good" or not, folks here are often willing to look at links/specs and share their infinite wisdom.