In A Tale of Two Cities, why does Jarvis Lorry pretend that Dr. Manette's illness has happened to a fictional "friend"?
I think you are referring to the scene when Mr. Lorry goes to speak with Dr. Manette after his recovery from a relapse to the amnesiac condition he suffered during and immediately after his imprisonment. The relapse was triggered when, on the morning of the wedding, Darnay revealed to Mr. Lorry his true name: Evremonde. The shock of learning Darnay's true identity sent Manette out of his wits and back to his cobblers bench for ten days.
Once Manette seems recovered, Mr. Lorry wants to talk to his old friend to convince him to get rid of the cobblers bench. Mr. Lorry feels that if the cobblers bench had not been available, perhaps the attack might not have happened. The cobblers bench is a symbol of Manette's imprisonment. Mr. Lorry does not want to confront Dr. Manette directly for fear of causing another attack or undue emotional upset. So he distances the conversation by using the artifice of a "friend." The tactic works well. By the end of the conversation, Manette has deduced that they are talking about himself and gives the advice to remove the offending "blacksmith's work" from the premises, which Mr. Lorry does after Manette leaves to meet Lucy and Charles. By distancing the conversation through the imaginary friend, Lorry gets Manette to see the problem more clearly than emotions would have allowed in a direct conversation.