#1 Do you see LED activity on front of router for the corresponing port to his computer?
#2 Move to another port on router or swap CAT5 cables. Use your CAT5 on his computer ( since yours is working )
#3 I would double check NIC settings.
Auto DHCP, Auto DNS etc.
sidenote. Sounds like maybe the NIC has took a dive, would maybe explain its MAC disappearing.
#4 Look into dumping TCP/IP and reinstalling or just reinstalling the NIC
#5 Unplug and curse the router and computer for 5 minutes and plug back in. GUARANTEED NOT TO WORK BUT IT HELPS YOU OUT!
Unless you changed default settings in router look for a 192.168 IP address