Elizabeth decides to tell Jane the truth about Wickham, but doesn't tell her the part he played in her relationship with Bingley. She doesn't want to hurt her, but it also makes it easier for her to accept her relationship with Darcy. Elizabeth and Jane decide not to tell the truth about Wickham to anyone because it doesn't really affect their social circle and he was leaving the next day.
In Volume 2, Chapter 18, Lizzy tells Wickham that she had just spent 3 weeks at Rosings with Colonel Fitzwilliam and Mr. Darcy. She tells Mr. Wickham, "When I said he had improved on acquaintance, I did not mean that either his mind or manners were in a state of improvement, but that from knowing him better, his disposition was better understood." She is letting Wickham know that she doesn't believe what he had told her about the feud with Darcy.
Reference: Thornes Classic Novels: Pride and Prejudice by Jane Austen. Stanley Thorne Publishers, England, 1997.
Lizzy said that he was his friend.