Then he has a Gateway most likely. Because you just answered your own question, by stating he has a wireless router. Just need the SsID & Passphrase, which are printed on the label of his Gateway. If he has changed the Passphrase & SSID from what ATT sets on the Gateway. You would need to get that info from him.
If it is low signal quality around the house. He just needs an Access Point, that uses modules that plug into electrical outlets, to allow extending the signal around the house.