She remembered also that, till the Netherfield family had quitted the country, he had told his story to no one but herself; but that after their removal it had been everywhere discussed: that he had then no reserves, no scruples in sinking Mr. Darcy's character, though he had assured her that respect for the father would always prevent his exposing the son. Chapter 36

Elizabeth does know of one other who had heard at least part of the tale. How much Wickham related to Denny is unknown to the reader, but he does know something.

the absolute fact of his absence was pronounced by his friend Mr. Denny, to whom Lydia eagerly applied, and who told them that Wickham had been obliged to go to town on business the day before, and was not yet returned; adding, with a significant smile --

"I do not imagine his business would have called him away just now, if he had not wished to avoid a certain gentleman here."

This part of his intelligence, though unheard by Lydia, was caught by Elizabeth, and as it assured her that Darcy was not less answerable for Wickham's absence than if her first surmise had been just
chapter 18

It could be that Wickham's story would have been everywhere discussed even if the Netherfield party hadn't left just after the ball. (This is pure speculation on my part)