Make sure you get a definition of God upfront before any discussion happens. Find out whether she thinks God is omnipotent or not. If she says he is, hit her with the Problem of Evil. There is no escape, she will be forced to admit that either God is evil or not all-powerful.

If she will accept a limited God, first point out that you are not talking about the God of the Christian Bible any more. Then ask for her evidence that God exists. She'll try to go circular on you, but be ready for that.