dudley is a wizard....thats right, he saw those dementors, dumbledore put somekind of magic barrier on dudley when he was young, that was the deal petunia made to let harry stay there, if dumbledore took dudley off the hogwarts registry harry would have a safehouse. Consequently dudley finds out and becomes infatuated with the dark arts, the new book ends with him killing his parents and basically the beginnings of him as a new dark lord