If his router has been manually reset, or came with default factory settings out of the box, it will assign his network adapter an ip address ... usually 192.168.0.2 or 192.168.1.2. Just be sure the network adapter is configured to obtain IP automatically or you've manually entered in the IP info of the router's subnet ... if you know it.
ISP doesn't necessarily need to be configured in order to network a couple of computers in an office or home, but does if you want those same computers to access public internet.
