On mugglenet.com is the Bloomsbury chat transcript that jk rowling participated in on July 30. We'll never know who because she changed her mind and cut that little piece of plot. Oh well, I still think it would have been Dudley.
Here is the relevant piece from the transcript,
"Snapedinhalf: You promised that someone will do magic late in life in book 7. I've now read it three times but cant work out who it might have been! Please help!!
J.K. Rowling: I'm sorry about this, but I changed my mind!
J.K. Rowling: My very earliest plan for the story involved somebody managing to get to Hogwarts when they had never done magic before, but I had changed my mind by the time I'd written the third book."