Mr. Lorry is speaking to Dr. Manette, the afflicted himself when inquiring about the illness which he delicately indicates has happened to a fictional "friend." Dr. Manette is in too fragile a state to discuss his own illness, but seems to be able to if it is discreetly attributed to someone else.
When Dr. Manette was released from prison after almost two decades, his mind was affected by the experience, and he was unable to interact with anyone, absorbed completely as he was in abstractedly working at his trade as a shoemaker. After his release, through the love of his daughter Lucie, he was able to recover and become something close to the intelligent, vibrant man he once was. When Lucie gets married and Dr. Manette holds conference with her beloved, Charles Darnay, Dr. Manette takes a turn for the worse and reverts back to his former malaise, separating himself from the community of others and compulsively spending his days exclusively tinkering at his workbench. Determined not to allow the good doctor to succumb to his illness completely again, Mr. Lorry watches him closely, and gently and carefully tries to bring him back to reality. When Dr. Manette gets to the point to where he will talk to Mr. Lorry again, Mr. Lorry astutely asks him to speak about the causes and cures of his own illness by asking about it indirectly, as if it happened to a "friend." The doctor is able to handle this non-confrontational method of communication, and tells Mr. Lorry what has brought the relapse on, and that his malaise seems to be abating (Book the Second, Chapters 17-18).