He should have installed it to the TC not the Mac..
If the cable modem is a router simply bridge the TC.. but usually the cable modem is a straight modem and it will not register the TC until you force a disconnection and renewal of the IP address.
So shut off the modem.. power down.. if battery backup .. remove the battery.. even unscrew the coax cable.
Wait 15min.. then put whatever you have done to power it down in reverse and power up again. Wait 2 min and then power up the TC.. make sure ethernet is connected to the WAN port of the TC.
Using the airport utility manual setup, on the summary page make sure you have the public ip allocated now to wan.. if not you have to repeat the process for longer periods.. 1 hour , overnight.. if it doesn't work, ring ISP and ask them to reset the port.
You may need to reset the TC as well.
If you have a modem router from Comcast it is a different issue.. details and exact model number helps.