I believe he would accept evidence if it demonstrated consistency
By ‘consistency’, do you mean ‘impartiality’?
and not just the use of the bible as the only historical account.
Does he say that the Bible
isn’t a historical account? (Sure, it’s an account from within the Church, but when we look at the New Testament, is he really going to claim that it isn’t telling the truth about the existence of Christ, the existence of the Church, and the content of Christ’s teachings?)
So for instants, any documentation from Rome about the Christians, i think he would recognize as substantiating the existence of the early Christians.
So, you’ve got a very new religion – in fact, one that was illegal because only ‘established’ religions were considered legal in the Roman Empire – and a relatively small movement which was centered in a backwater corner of the empire. What,
exactly, does he think that Rome would document about Christians?
In any case, we see Justin Martyr writing in around 155 A.D. to the Roman emperor, in response to the claims being made about Christians in public discourse. Does that not prove the existence of the early Church?
If the Romans had documentation of Jesus or any of the disciples, again, I think he would take and accept that.
Again, how reasonable is that request? The Roman Empire in the first century A.D. was a very different time and place than the West in the 21st century. No widespread literacy. No documentation of individual citizens in central repositories. No popular media in the way we think about it today, and certainly no
national media.
So, I think it would be incumbent on your friend to explain why he thinks that the things he’s asking for are, in fact, reasonable to expect to find. And then, when he realizes that they’re
not reasonable expectations, perhaps he’ll revisit the conclusions he’s drawn from their lack of existence.