It won't cost you anything on 48.
If you are not on wifi it will use some of your data allowance.
You say that her contract charges her for using FaceTime?
That sounds odd.
She needs to check with her own provider about what charges apply.
I would have expected it to cost her nothing if she was on wifi.
If she was using carrier data then that may cost here but it depends on her carrier and contract.
Bear in mind that FaceTime may need activation on the 48 network due to the stupid way that Apple do things.
So if you haven't already used FaceTime on 48 then you may need to do this:
Hope this helps.