So let me just preface this by stating that I have decided to reread the Harry Potter series for at least the hundredth time; but this time, I am reading them in reverse order.
That being said, in Order of the Phoenix, A Peck of Owls, we have the following, emphasis is my own:
"De-ment-tors," said Harry slowly and clearly. "Two of them."
"And what the ruddy hell are dementors?" [Uncle Vernon]
"They guard the wizard prison, Azkaban," said Aunt Petunia.
Aunt Petunia looked quite appalled with herself. She glanced at Uncle Vernon in fearful apology, then lowered her hand slightly to reveal her horsey teeth.
"I heard – that awful boy – telling her about them – years ago," she said jerkily.
Harry Potter and the Order of the Phoenix, pages 31-32, US Edition
Now Harry takes for granted at the time, and understandably so, that "that awful boy" was in reference to his father, James.
Fast forward two years to Deathly Hallows. When Harry views Snape's memories, we find that Lily and Snape were friendly as children, and that Snape was the first to actually tell Lily that she was a witch. Petunia, as we know is jealous of this fact, and takes to spying on Lily and Snape:
"They wouldn't give you to the dementors for that! Dementors are for people who do really bad stuff. The guard the wizard prison, Azkaban. You're not going to end up in Azkaban, you're too –"
He turned red again and shredded more leaves. Then a small rustling noise behind Harry made him turn: Petunia, hiding behind a tree, had lost her footing.
Harry Potter and the Deathly Hallows, pages 667-668, US Edition
So we know for a fact that Petunia did overhear Lily and Snape's discussion about dementors. That is not to say, however, that she never heard Lily and James talk about them.
So my questions are:
Is there any canon evidence that states about whom Petunia was referring to as "that awful boy" in Order of the Phoenix, or is this all just a massive coincidence?
Is JKR really that organized to have planned out such small details that far in advance?
Why am I only making this connection now? [This question is more rhetorical than anything else]
Any canon-based answers, or answers in the spirit of canon, would be much appreciated.