I really do not think this would work by making the IP the one provided by the ISP.
Main thing being his xbox is connected to a hub, not a router, so he would be using Internet Connection Sharing, making it a non local IP would stop the PC from seeing the XBOX, and therfor the xbox will not be able to see the internet.
Does anyone know if said techinque would work even with a roughter, i really have my reservations.
What you want to do is look into (Google) network tunneling software, he would run it on his PC and you would run it on yours, effectivley stitching both your networks together (meaning you would probably have to have compatable network settings (Workgroup name, IP addresses....etc))
Do not confuse, Network tunneling software with Xbox network tunneling software (e.g. XBConnect & Xlink) theese use the same principles, but are designed for system link games only.
- Shorty