I dont think that there is an easy way of doing it...
You will need some kind of device or software sitting between him and the outside world. Is there not anything on your router that can do it?
There are plugins for things like ISA firewall
http://www.policypatrol.com/Web-filter.htm#User_quotas
But I think that might be a bit over the top somewhat!
There are also things like this
http://bandwidthcontroller.com/index.html
But you need to have that installed on the machine, which is a bit obvious and I suspect you want it transparent?
Not sure if anybody else out there might know of anything?
Can you not just ask him to limit it? So that you are not effected?