If you are putting a router in the arrangement then the whole thing needs to be run over ethernet, you can no longer use the ethernet port on the modem.
You buy a router (the linksys WRT54G for wireless or BEFR41 for wired are good, as are others). Plug the Modem into the WAN port of the router and the PC into one of the four other sockets (with the extra cable provided). That should mean that you now transparently have a routed connection.
You then need (if their PC does not have one) a ethernet card on their PC and a cable from that to the router. Alternately buy a wireless ethernet card for their computer and connect wirelessly. Putting these cards in a PC is a lot less daunting than it sounds if you need to do such a thing.