well, this problem might be solved a little easier with a bit more information.

possibilities include conflicting IP addresses, which happens to be among the only thing i can think of at the time.

a good way to solve a problem like this would be to get a router. they usually price in the $100 range, or maybe a little cheaper in the states. they're not hard to set up, and you just have to make sure there is a NIC in each computer and enough CAT5 cable to go from the router to each computer, and the router be close enough to the Internet connection so you can plug it in. for configuring a router, just put the dics in and follow directions. easy like pretty much every other software installation. after that, you just set private IP addresses on each of the computers (10.x.x.x, 192.168.x.x, 172.16.x.x, etc..), and you're all set. the router will take the original IP address given by the ISP. if your friend gets a static IP address, then you probably will have to set IP addresses for each of the computers. if it's a dynamic address, then the computer might automatically configure to use DHCP.

the only problems that i've found with some routers is that every now and then, they might cut off one of the computers. mine signs me out of ICQ or MSN Messenger at random intervals, so i'm constantly signing in and out. it's inconvenient, sure, but it's better than paying the extra money per month. plus, i love networking, anyway.

but before you go out, buy a router and a bunch of other networking gear, make sure that you find the right problem, anyway. this is just one quick fix.