How can I help my friend?

Today my friend’s ex-boyfriend just spread a nasty rumor about how she slept with him. Then he told her parents as well and, surprisingly, they believed him. Which is ridiculous because one, she never slept with him and two, they should believe their own daughter over some random boy.
Anyways the fact of the matter is that her parents believe him over her. Because of this, she is being forced to move to her mom’s house, which is 350 miles away. So I’m wondering what I can do to help her.
I thought about trying to make her ex tell the truth, but I don’t know if I can convince him. He is pretty stubborn and I don’t know him that well.
So any ideas?