My friend is beyond believing Jesus was God, she insists that He never even existed as a person, and was just a made up figure.
It isn’t enough for me to say that it’s irrational to think the biggest religious movement in the history of the world was started based on a man his contemporaries would have known was fake. And obviously the Bible doesn’t do anything for her.
Is there any hard historical evidence and atheist would accept for Jesus living, supernatural stuff aside? Any a-religious ancient historians who wrote about a prophet in the area who was executed? Would the Roman empire have kept a record of prisoners they executed? Anything like that?
There is plenty of hard historical evidence that Christ existed if you apply the same historical standard of evidence that you use for any other historical figure. Unfortunately your friend is either purposefully denying the use of the same historical tools, or is ignorant to the methods of historians.
The way we know of pretty much any figure in ancient history is primarily from the writings by or about the person in question. So for example, we know of figures such as Pericles and Alcibiades primarily from the writings of Thucydides. We know of many of the figures in Roman history from the writings of people such as Tacitus and Suetonius.
We actually have a wealth of biographical material about Christ, that was written by his contemporaries and later historians. The gospel accounts, each of which purport to be historical biographies of the ministry of Christ, provide us with very close contemporary accounts of the life of Christ. Even the later dates for the gospel accounts are within one generation of Christ, providing pretty reliable evidence when compared with other characters we accept as historical figures. We also have the writings of Paul who references living witnesses to the ministry of Christ, naming some of them by name. But besides them, we have hundreds of writings by patristic fathers. Not only that, but we have writings from secular historians such as Josephus, Tacitus, and Suetonius that admit the existence of Jesus and that people followed after him. There are even Midrashic references admitting to the existence of Christ. The biographical material for Christ is probably richer than any other figure in ancient history.
Direct archeological evidence for a historical figure is quite rare. So for example, we have no direct archeological evidence or the existence of Alexander the Great, we rely solely on the written accounts of his life. There are however a few pieces of direct archeological evidence of Christ. So for example, you have the James ossuary, which was at first looked at skeptically by scholars, but is becoming more and more accepted as an authentic piece of archeological evidence directly linked to Christ. They also believe that Peter’s home in Capernaum has been found.
In other words, if your friend will allow for the use of a reasonable standard of evidence, then there would be no justification for denying that Christ existed.