Without a MODEM he can't connect to the internet so why exactly would he split his cable? More info:
A single-box experience, sometimes called a Gateway, comprises TWO separate components:
1) MODEM (to connect to the internet), and
2) ROUTER (to connect multiple computers/devices to each other AND to the MODEM)
A ROUTER can work on its own to connect home devices together but you'd have no INTERNET connectivity. For that, you need a DSL MODEM.
*ROUTER's look identical to "Gateway" devices that contain MODEM's and are standalone. Again, the ROUTER alone needs a separate MODEM.
When you buy the DSL MODEM, you then plug the Ethernet Cable into both it and the ROUTER, then connect your wired devices to the Router (or setup wi-fi with the router).
The RJ-11 phone cable goes from the MODEM to the phone jack.