I understand your point, and sometimes restarting the DNS Client service on an XP workstation will clear things like that up. However, he's able to connect to sites, but not access certain functions on those sites - which to me sound like either a firewall issue or a caching issue. Without some reply by him, though, we're all shooting in the dark.
I'd turn off the firewall and see if he can replicate the errors. If not, it's the firewall.
If the errors are still evident, clear the page cache (Tools > Internet Options > Delete Files). Close and re-open all browser windows and try those pages again.