JK made the rare decision to let someone read her book before it was published when she heard that Catie had cancer.
She also sent Catie emails and presents before she died on 18 May 2000, the Sunday Mirror newspaper said.
Catie, who was a huge Harry Potter fan, had just got to the end of the third book when she found out she had weeks to live.
Catie's mother, Gina, told the paper: "I will be forever grateful for what Joanne did. She gave us something priceless by having this relationship with Catie."
JK Rowling earned more money than any woman in the UK in 2002, it's been revealed. She made £48 million from the Harry Potter phenomenon. She even earned more than the Queen, Madonna and Kylie!