You've come to the right place for an answer...
Go to
this page on Shreve Systems' web site and scroll down about half way. It looks like they are selling a 10 megabit ethernet card for the COM slot, and it looks to be compatible with the Performa 630 series, amongst other things.
You'll also find some other interesting stuff on that page. Also check out the rest of Shreve Systems' site; they specialize in old Mac hardware. Who knows what else you might find.
What I would strongly suggest is to backup whatever is on that system that he wants, then nuke that sucker with either the supplied CD, or a System 7.6.1 boot CD. After you've installed the drivers for that card (if there are any), your nice, clean system should also have all the necessary system components installed.
Go to Control Panels > TCP/IP and make sure that card is chosen as the network adapter. After that, turn on file sharing, share one or more folders or share the whole hard drive, and when you check the network from the other end, good Lord willing and the creek don't rise, this computer should show up. I'm thinking you might need to enable AppleTalk on the G5 under System Preferences > Network > (pick your current adapter) > AppleTalk tab > Make AppleTalk Active.
I really haven't dabbled too much myself with Mac OS Classic <-> Mac OS X networking, but I think Apple tried to make the support fairly backward-compatible.
Good luck!