I've wondered about Denny before. IIRC, he knew Wickham before W. joined the Regiment. Early in Ch.18 when Wickham didn't attend the Netherfield Ball, Denny informed the Bennet girls Wickham had been obliged to go to town on business 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.' If Denny was able to reveal W's plan to Colonel Forster he either knew it from W's confidence or at least knew Wickham's real charecter. Denny couldn't control W's behaviour but if he was informed of the plan, then he's at fault for not warning Colonel Forster sooner as Wickham was under Forster's orders. Denny seems a slacker, probably not so bad as W. though as fellow-officers they would have shared the occasional low life. He is at fault for not blowing W's lies too. I wonder if it was arranged Denny 'betray' Wickham so Lydia's family would know he didn't intend to marry her so W. could exact his price.