Horses no matter how good or well trained if they think they can do as they will will do just as they want. This is why I disagree with people who say they have a partnership with their horse and they ask the horse to do something.
You need to get her to under stand that she is the leader and as long as she leads with under standing and authority and not as a dictator the horse will respond to that and will follow her where ever it is she wants to go.
Horses like any pack or herd animal needs structure and leadership. If you (human you) can provide that they will respond.
Hope it all works out and the horse comes around and works well for her.
I had the vet out and he said the teeth are fine... so I think you were right. She is letting him take charge so to speak. I also noticed that now when she rides she is holding her reins tighter, like maybe she is scared that he is going to run again. Hope the lessons help, he is a great horse.