(Book 2 Chapters 17-20) What request does Lucie make to Darnay regarding Sydney? Why would she ask this?

Carton leaves. Darnay speaks generally of the conversation at dinner, remarking on what an odd and dissolute character he is. Darnay means no harm and is only speaking the truth, but later that night Lucie implores him not to speak of Carton in that way but to feel some sympathy for him, which Darnay readily agrees to do.