He didn't buck. He gave a big kick and turned out from under her. If she had kept his neck bent while she put leg on him he couldn't have done that. If I were helping her I would tell her to keep his head bent to one side or the other and not ride him in any straight lines when he is tense in his face, neck and back.
Uhh.. kicking out is the same thing as bucking. Unless you can prove they are two completely different things, please keep your comments to yourself.