Did you switch the modem off when changing cables? As this may have worked it's magic in detecting the pc correctly. Technically it shouldn't have made anything work at all. But hey.
Glad you got it working
Actually, did he test his laptop to the modem to check the connection before he left? If he did, the modem will have "learned" his laptops unique MAC address and held onto it. when you switched it off, it forgot this, and instead went onto yours.