On her computer, run ipconfig to find out if it is a static IP address or DHCP address. Then ping the IP address. Then run arp to see who has the IP address. You may need to do this over a period of time, but you should eventually get the MAC address of the other computer using her IP address. Then figure out how there is a conflict (dhcp should handle this without assigning the same address). Probably some device has a static IP address or there is another unauthorized dhcp server on the network.