On HIS pc, which i assume is the internet gateway, you need to disable the ability for other computers to control the connection. This will mean, however, that you will have to manually connect his computer to the net when you want online.
To do it is pretty simple.
(Based on XP Home)
Go to All connections
Right click the shared connection (will have a hand under it)
at the top click advances
uncheck the box with "Allow other network users to control......"
Should do the trick.