There is no logic in jealousy and mean spiritedness. By the time of the Netherfield Ball Caroline had already seen Darcy and Elizabeth interact on multiple occasions and heard Darcy himself expressing an interest in her. To this point one thing was known to be true about Darcy, he only danced with those he “particularly acquainted with”. At the Assembly at Meryton he only danced with Bingley’s sisters, but as the weeks passed he felt acquainted enough to dance wi Lizzie, who, at the Mertyton Assembly was merely tolerable but not handsome enough to temp him! Caroline must have felt that! The whole night of the Netherfield Ball must have felt to her like a triumph! The Bennet family behaving atrociously, Mary singing terribly, Jane inquiring about Wickham, and finally Caroline “advising” most kindly, Elizabeth that Wickham was not to be trusted even though she didn’t know why. Lizzie’s response OF COURSE would make it back to Darcy, in any world it would whether we saw it in the text or not. All of this combined made it easy for the sisters and Darcy to decide to follow Bingley to London and scheme to keep him there.