I read a thread somewhere...on A harry potter discussion forum but i cant remember which one!

It was talking about how when Petunia blurted out that she knew what Dementors were and what they did...she said that she heard "that horrible boy" tell Lily about them

We all assumed "that horrible boy" meant James, but it didn't! It meant Snape!

Snape told Lily about Dementors that same day that Petunia was found spying on them. She didnt like Snape either...so "that horrible boy" fits