Disclaimer: Much of this belongs to J. K. Rowling. Since this is an alternate scene, some of the lines even belong directly to her. This is an unauthorized fanwork.
"And what the ruddy hell are dementors?" Uncle Vernon demanded, clearly not believing a word of Harry's explanation of what had caused Dudley to return to the house sick and nearly mute.
"They guard the wizard prison, Azkaban," said Aunt Petunia.
Uncle Vernon was goggling at her. Harry's brain reeled. Mrs. Figg was one thing—but Aunt Petunia?
"How d'you know that?" he asked her, astonished.
"I heard—that awful boy—telling her about them—years ago," she said jerkily.
"If you mean my mum and dad, why don't you use their names?" said Harry loudly.
"Of course I don't mean your dad," Petunia snapped. "I mean that horrid Snape boy."
Harry was so astonished that he almost forgot how angry he was at the world. "What? You know Snape?"
"Don't ask questions, boy," Uncle Vernon snarled, but the shock of dementors in Little Whinging seemed to have changed Aunt Petunia's attitude as nothing else had in fourteen years of living with Harry.
Aunt Petunia sat stiffly down at the table and began speaking woodenly, staring at Harry without blinking and without seeming to see him. "Of course I know Snape. He was the one who told Lily she was a witch. Lily and I were playing on the swings at the play park and then he showed up and told us. He said he'd been watching her for ages, like spying was some sort of compliment. Oh, he was a slimy little boy from down on Spinner's End and I told her to have nothing to do with him and his baggy jacket and frilly blouse. But of course she didn't listen. She abandoned me and spent every spare moment with him, and look where it got her."
If Harry's brain had been reeling when he'd discovered that Aunt Petunia knew about dementors, that was nothing to what it was doing now. Snape had told his mum she was a witch? Snape had been her first friend in the wizarding world? It was tempting to think Aunt Petunia was lying, but then, how would she know Snape's name?
Before Harry could pursue this train of thought further, Uncle Vernon broke in. "I'm interested in what happened to Dudley! If these dementy-whatsits really do exist—"
"They do," Harry interrupted, on firmer footing now that he had something to think about other than Aunt Petunia's surprising information about his mum. Aunt Petunia nodded.
Just then, the third owl of the evening zoomed through the still-open window like a feathery cannonball and landed with a clatter on the kitchen table, causing all three Dursleys to jump with fright. Harry tore a second official-looking envelope from the owl's beak and ripped it open as the owl swooped back into the night. As Uncle Vernon grumbled about owls in his house, Harry scanned the letter and found to his relief that he was not definitely expelled, although his relief was tempered by the fact that he was to have a hearing in ten days' time to determine his punishment for using magic while both underage and in the presence of a Muggle.
The rest of the night full of arguments with the Dursleys, but when at last Harry was banished to his bedroom, he couldn't help mulling over what Aunt Petunia had said about his mum. If Aunt Petunia had known Snape, then Snape had known Aunt Petunia . . . for a moment, Harry wondered whether that acquaintance was secretly half of why Snape hated him. He'd be inclined to hate someone, too, if he knew they had such awful relatives as Aunt Petunia.
But no. Snape, Sirius, and Dumbledore had all given Harry the unshakable impression that there had been something bad between Snape and Harry's dad. With this new information about his mum, though . . . an awful thought struck Harry. If Snape was Lily's first friend in the wizarding world . . . could he have been jealous of James for marrying her? Harry shuddered. Surely not—surely Snape had merely hated James out of arbitrary nastiness, the way Malfoy hated Harry.
And what about Lily? Had she liked Snape at the beginning just because he'd told her about the wizarding world? That certainly was a fantastic secret to be let in on, a fantastic world to join, as Harry knew well. But could there have been more between them? Surely not. And yet . . .
The next three days stuck in his room gave Harry an unwelcome amount of time to think without distractions. By the time the Order of the Phoenix rescued him, he was more disgusted with Snape than ever. This would be a bad year in Potions, no doubt about it . . . but anything had to be better than staying cooped up at Privet Drive.