If she's using DHCP, her DNS server is probably set via DHCP. So I can see two possible problems. One, she accidentally has her DNS servers set manually and incorrectly. If that's the case, change the setting back to use the DHCP assigned DNS server. Two, for some reason, the DHCP assigned DNS server isn't being used properly. So try setting it manually.

The DNS server location moves around in Windows a lot, so just go searching for it. It should be in a network control panel somewhere.

These are just guesses, as the symptoms you describe don't make a lot of sense.