Your friend suffers from the illusion of deism. The Catholic Encyclopedia explains:
Deism not only distinguishes the world and God as effect and cause; it emphasizes the transcendence of the deity at the sacrifice of his indwelling and his Providence. He is apart from the creation that he brought into being and unconcerned as to the details of its working. Having made nature, he allows it to run its own course without interference on his part. In this point the doctrine of deism differs clearly from that of theism.
The Catechism of the Catholic Church describes deism this way:
Some admit that the world was made by God but as by a watchmaker who, once he has made a watch, abandons it to itself (CCC 285).
To deists, prayer is futile because they think God is not involved with the world he created. But since your friend concedes that God may exist, you have a common belief from which to start. Explain that Christians believe God revealed himself to mankind so we can know him and interact with him. Eventually, he came to live with us for a time as the man Jesus who taught us how to pray. A couple of resources that may be helpful in getting started are Handbook of Christian Apologetics by Peter Kreeft and Ronald Tacelli, S.J., and Mere Christianity by C. S. Lewis.