It was very kind of you to help, but horses that have gotten away with murder need professional help. You're likely right that she isn't helping the situation, but having said that, it's easy to do if you don't have the experience to deal with a hard-to-handle, or challenging horse. Advise her to get a trainer, or sell the horse to someone who can handle him.
It honestly sounds like you might benefit from a few lessons as well, and I'm not saying that to be mean- I take lessons as much as I can because it helps me improve, and have another viewpoint. Exercises, and changing things up will keep your horse's attention on you. Ride serpentines, circles, change directions, set up obstacles or poles, and set a goal to learn something new.