it is a 24/7 operation.
I suspect that she is not actually reading the instructions that came with the Airport Express.
When she configures the AE, she should be telling it to establish a wireless network.
The layout should be, Incoming Cable to Router. Airport Express plugged into an Ethernet port on the Router.
I didn't think that Comcast would allow you to to change out Modems, they don't in my neck of the woods
P
Hi, My friend recently got a Mac desktop computer and had a netgear router with a comcast motorola modem. Well she wants to stream netflix to a bluRay player so she got a new Motorola Extreme Surfboard modem and the apple guy at BB talked her into an Airport Express router instead of the Netgear N600 Wireless (and she's also going to get a wireless Sony BD). Well she got the modem setup nicely and says it's faster. But she can't get the Airport Express to work. It says no valid ip address. She called Apple care but they were closed. I don't know apples and I'm not there to see her screens. I'm not even sure she has the cords plugged in right.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic