Complex sentence: Garson, in turn, would then privately coach the lawyer as to what questions he should ask of witnesses in court in the case before Garson, and what arguments the lawyer should make to Garson in court.
Split into simpler sentences: Garson, in turn, would then privately coach the lawyer. He would tell him questions the lawyer should ask of witnesses in the case before Garson, and arguments that the lawyer should make to Garson in court.