Go into System > Administration > Networking and make sure all the settings are correct. The ethernet connection should most likely be set up as DHCP (there are of course other ways of doing this, but my setup is like this). Next, log into the router and make sure all the settings in the router match up with the what is trying to be accomplished. For example, you'll want to make sure the router is asking for an IP dynamically (again via DHCP), the DNS settings should also be getting grabbed from the ISP. If you've set up the settings statically (only allowing a specific block of IP addresses) then make sure that one of those addresses correspond to the IP address range offered (that you set on the router) and set one of those IP addresses on your (his) Ubuntu box.