This line can be found on Page 159 of the British edition of the book.
Natalie McDonald is a real person.
And this is her story.
In July 1999, Pottermania was blowing up all over the world. J.K. Rowling was, in the words of a 2000 article in Maclean's, a Canadian magazine, "refusing all media requests and most outside distractions, as she worked feverishly on the lengthy story that eventually became the 636-page Harry Potter and the Goblet of Fire."
Meanwhile, in Toronto, 9-year-old Natalie McDonald was dying of leukemia. A family friend, Anne Kidder, would later say: "She was obsessed with the Harry Potter books. They had been her respite from the hell of leukemia. And because I'm the sort of person who thinks there must be something I can do, I badgered Rowling's publishers in London, sending them a letter and an e-mail and a fax for her."
But the story doesn't end there.
Natalie's mother Valerie wrote back to Rowling.
She said: "That letter touched deep. I just knew, reading it, that if we had been two mothers waiting for our kids at the school gate we'd have been friends." The next year McDonald, her husband, and their two daughters travelled to Britain to meet Rowling. And it was there, reading the book on the tube, that Valerie found out about the gesture in Rowling's book.