From my experience with ethernet networking:
1. Is the light on at the switch/hub/router indicating which port his cable is plugged into?
2. Is the light on at the back of his computer indicating a connection (usually an orange light) ?
No to either of these = try a different port in the hub/switch/router.
If different port did not work, try a different cable, his cable may be damaged.
If different cable did not work, try installing a different network card in his computer.
The rest of these points assume that the lights on his computer's network card work and that the light on the hub/router/switch for his port shine brightly.
3. Does his computer have an ip address? (Command prompt : Type in ipconfig)
No : Try going into network settings -> network card -> properties -> tcpip and a assign his computer an ip address manually.
4. Is his ip address within range of the other computers on the network's ip address?
e.g. If your computer is 192.168.0.3, then his ip address must be 192.168.0.4 or 5 or 6 or 7 or ... 254.
or
e.g. If your computer is 10.0.0.3, then his ip address must be 10.0.0.4 or 5 or 6 or 7 or ... 254.
If it is not, then manually give his computer an ip address of the correct range.
5. Are all computers using the same TCPIP default gateway?
6. Are all computers allowing Netbios over TCP?
This setting will allow computer names to be identified a lot easier by other computers on the network in the absense of a domain controller.
7. Try disabling his firewall.
8. Try disabling your firewall at the same time as step 7.
If this does not help, well, I pretty much give up. Good luck though.