As long as she can dial a US number or toll free number (ex: 1-800, or 1-888, etc) to dial the international number it will be fine. You will pay for minutes used, that's all. We do this on our family plan and it saves a lot of money. You could technically even get on a Friends and Family plan and add a non toll free number in one of the slots and the minutes are free. Nights/Weekends are also free if you are on one of the F&F plans.
But if you are outside of the US then its going to charge you the cost of minutes @ roaming rate.
We use TDS