Just in case it might be helpful to anyone else, I'll repost my original response:
You can refute this claim very easily with Scripture alone:
The angel explicitly states to Mary, "You will conceive in your womb and bear a son..." This leaves no doubt as to the authentic motherhood of Mary. A surrogate mother carries a child who was already conceived outside of her. The angel tells her that she will conceive a child.
Further, Jesus had 2 natures: divine and human. God provides his divine nature, Mary provides his human nature. In order for Jesus to fulfill his role as the "new Adam", it was necessary that he have an inherited human nature. If the Father had simpy formed the Son ex nihilo (from nothing), He would not have any relation to the human race He came to redeem.
You should also explain to your objectors the doctrine of the Immaculate Conception, as I'm sure they believe Mary was a sinner and thus could not be the real Mother of God, and also explain that it is a doctrine that has been held since the beginning of Christianity. You can find many writings by the Early Church Fathers supporting this tenet of faith. If you are not thoroughly familiar with it, I'd be happy to expound upon it.
The burden of proof is on your friends, and, unfortunately for them, the evidence from Scripture and Christian history is against them.