First off, don't call anything your girlfriend's mom has "stupid;" it tends to cause relationship problems.
What you really want to do is to get a wireless router. These are one box that includes both a cable/DSL router and a wireless access point. You plug the WAN port into the cable modem, the existing computer into one of the LAN ports, and configure the wireless part to work with your PB. The box isn't too expensive, it will protect the GF's mom's computer from the bad guys in the outside world, and it will help you out too.