Tell her this -
The person who spawned all of the conspiracy theorism that inspired
The Da Vinci Code made it up in his basement in the mid-20th century. He wanted to prove that he was descended from the Merovingian line (main nobility who ruled what we now know as France back in the day) so that he could rule France, and in order to make the Merovingians seem extremely important and royal, he made up a bunch of stuff about how the Merovingians are related to Jesus. Then he could say, "I am descended from the man they call the Christ! I deserve to rule France!"
I am not joking. There was a tv show about the guy, and they showed an interview with him. I apologise, but I cannot remember his name.
It was weird and he was really wacko.
But his ideas got the conspiracy theorists going and BAM, now it's "cool" for people to think that this stuff is true because it gives them a reason to feel superior to those dumb Christians.