Help Fixing a Friends Computer?!?!

Your first option is to swap Operating Systems?? Seems pretty drastic to me. If it was working fine on his Dell before, then maybe a driver update did come out that he hasn't picked up yet. I'd try that first. If not, maybe the cable went bad. Try a different cable.

You can create an Ubuntu live CD that doesn't require you to make any changes to your computer (An "installation" doesn't imply altering drive content per se). I'm assuming he has already installed and/or updated the network adapter drivers to no avail since it's probably a more obvious thing to attempt than to try to find help on a forum. If so, using my approach is a fairly painless way to detect hardware errors even for fairly obscure devices.

Anyway, ensure that you're using the proper EIA/TIA-568 cable standard. TIA568A to TIA568A or TIA568B to TIA568B network cables are probably what you're looking for. If you have no clue what I'm talking about, just try a few different ones since crossed Ethernet cables (TIA568A to TIA568B or vice versa) are not as common. If you're using the wrong type, any attempt to establish a connection just "won't work" for some seemingly inexplicable reason.

Oh, and if your friend has never established a connection on his system before, ensure that you're using the appropriate network settings. Some ISPs don't run DHCP services, so you'll have to set your IP address and gateway manually.