Assuming that your friend is a non-believer (like me) I would advise pointing out that while it is impossible on the basis of material evidence to prove that the virgin birth took place neither is it possible to establish that it did not.
That is because the assertion cannot be tested. It is said to have occurred but it was never observed in a scientific way and cannot be replicated. So it’s a ‘faith belief’ and beyond argument based on scientific observation.
If like all non-believers your friend assumes that Jesus was conceived like us all then there are a range of possible explanations as to why the scriptural account could have been written.
His or her guess is no more open to debate on a rational basis than the Catholic belief. The best a non-believer can say is: ‘miracles do not happen, therefore this did not happen. But some people said it did’. We don’t know why they said it did. They may have believed it, or told it as a ‘fitting’ way of illustrating their belief in Jesus’ divine nature, or they may have deduced it from their belief he was divine, thinking that if he was divine must have been conceived in that way.
Bart Ehrman, my favourite agnostic biblical scholar has pointed out one aspect of the story that is unique: although these are many, many stories of ‘virgin births’ of important or mythological figures in the ancient world, the story of Jesus is the only one in which a god does not actually have sex with the virgin. There is no other story, he says, of any virgin becoming pregnant without any direct involvement in a physical sense with a god.
