I don't think that is possible. Why not just use her Apple ID?
I don't see how that is any more convenient. FaceTime is pretty simple, open the app and touch a contact name. The process would be the same with the Apple ID or a phone number.
They can't use the iPad natively for normal calls. If they don't actually need data on it, might as well just cancel the data plan instead of wasting money every month.
What's the point of using the number for FaceTime/iMessage when the Apple ID works just as well? It's not like you have voice or SMS fallback anyhow. That said, iOS doesn't give you the option of using the iPad's number for FaceTime or iMessage.I'm not talking about making cellular calls on the iPad. I was asking if they can receive FaceTime calls using the number tied to the iPad. It seems that this is not possible.
The problem isn't with making FaceTime calls. They want to use the number to receive calls. They're paying extra for data and wanted to use the number since the data plan is pretty much useless for them as they don't bring the iPad out of the house.