This could be a problem with the PPPoE configuration (Chuck mentioned DSL). Newer DSL modems (e.g. the ones Verizon ships) include a PPPoE client (as well as NAT) and if he has a router trying to provide that functionality, they will both appear to malfunction.
Recently my Dad switched from cable to DSL and he was frustrated because he couldn't just unplug his wireless router from the cable modem and connect it to the DSL modem. I had to use the DSL modem's web interface to disable the NAT and PPPoE to make it a simple bridge, so that the wireless router could manage those aspects of the connection.
Matt
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