...what handsets you both currently have.
You *could* have her use Skype if you have a home computer and she can either load a Skype app on her smartphone (or PC). Other options such as VoIP apps exist too, but you have to get more specific about what she could do while abroad.
I have an ATT Talk Nation Family 550 plan for me and my daughter. My charges are $50 per month, but her's average $80 per month. She has a Smartphone with lots of addl. charges since she lives abroad. $1,500 per year for our two cellphones, that seems excessive. Any advice about how I can reduce these charges. I hardly ever use my phone, but of course she lives on hers.