It's true that virtually every major culture has a flood story in its folklore It's curious that there aren't any worldwide fire myths or global glacier tales mixed in...
Everybody talks about the flood, though. Maybe the best explanation is that there really was such a deluge it kept everyone talking for thousands of years...
even though some of the details got mixed up in the retelling. Does this seem like a reasonably possible explanation to you?
If the person asks where's the proof for God/Jesus/Christianity, go here.