Firewall maybe?
Hi,
If he has a firewall, there's a possibility that the settings have changed and it coud be blocking outgoing DNS.
Most websites would have static IP's but secure sites or high profile sites would change their DNS address every now and again. If DNS has been blocked then the DNS cache on his computer might be out of date.
I know Zonealarm, has a facility to block DNS traffic or to block a service that is running DNS for you(Services and controller Apps on Win2K and XP). Maybe this could be the problem.
Hope this helps.
Cheers!