Other mistake: Research in the end credits is misspelled "Reasearch."
Other mistake: When Mattie is going to get water from the creek, she is shown carrying a large, bulky, heavy wooden bucket. Had they been carrying this in their gear, it would have been visible, but this bucket is never seen among their equipment prior to this scene. Not to mention, it would have made much more sense to have used a canvas, collapsible bucket as they used in the original film.
Answer: It was Frank James, brother of Jesse. He didn't stand up as she approached. She was offended by his lack of manners.