Corporal Morrison + Martha Kellog (Spoilers*)
have a scene of a not inconsiderable length where they meet and he informs her that he had heard Custer instruct Benson to pick her up, thereby being in a position to assist the captain regain his honour with his brother officers.
She gets very excited and he then rides off on Custer's horse to deliver a message to Benson only to be killed by an indian, essentially making the whole previous scene fairly pointless apart from providing Custer's riderless horse for the climax at Little Big Horn.
When Benson returns from the mission, successful in the army's eyes, the questions surrounding his honour aren't even raised nor is any mention made of the corporal's proposed testimony.