if he uses a file sharing network like Limewire or even something like Bit Torrent you can disable the port forwarding if you are behind a router...I honestly dont know how to do that, I do however know its in the router options, if you log into the router you should be able to find it fairly easily, or just google, "Disabling port forwarding "you router here""
haha let me know if that helps, im sure more people will know better than I do, and If i was at home I would look into how do that on my router.
It's a free firewall. In the program, you can disable and enable each program's ability to access the internet. The simple way to do this would be to install it, then start up Limewire or whatever. ZoneAlarm will ask for your permission to let the program use the internet, and say no, and check the always use this option box.