Not sure if this router will do any throttling. How about you put your Roomies PC on a wireless connection then you can throttle back by using the Modes, try putting him on G mode only, if he's still hammering the connection, go to B mode only. Worth a try. G mode give 54Mb speeds, B mode would slow him down to 11Mb if needed.
You could still leave his lan cable connected however tell him to disable it when your gaming, and use the wireless. When your not gaming then let him use the Lan connection.
Let us know how this goes.