In the novel Towards Zero, Guiseppe Antonelli is a witness in the "Lamorne" case which had recently concluded in court with the acquittal of the person accused of murder. The case, and the performance of the legal personalities involved, the judge Bentmore, prosecution counsel Depleach and defence counsel Arthur was the subject of much discussion among Mr Treves' circle of legal friends in the prologue to the novel.
Mr Treves, widely regarded as the senior and most respected member of that gathering, mentioned Guiseppe Antonelli as yet another of a series of causes, circumstances and coincidences which led inexorably to the result of the case, "towards zero", as he put it.
Guiseppe had, it seemed, "come over" to exchange with his brother for a month. Presumably this was some kind of position as a domestic staff. Guiseppe had very sharp eyes while his brother was "blind as a bat". It was only because of this exchange and his eyes that Guiseppe saw something important to the case. Had the exchange not take place his brother would never have seen anything. The context of the conversation suggests that Guiseppe is a restaurant proprietor in Italy.