It is not possible to predict when she will actually menstruate next. She can either wait for that and start a new pack, or start a new pack when she gets back.
You should take extra precautions the first 7 days of the new pack. She can keep the left over pills from this pack as spares, incase she lose one. Sometimes a pill will fall and go missing, or go down the drain. These extra pills Are ideal to replace those.
Take care!