Another thought. If she is 17 that makes her a minor. Wouldn't these be her parents horses? Wouldn't they be the responsible adults? Maybe she is following their poor example. I agree with the suggestion to get a knowledgeable horse person involved, someone the parents would respect and listen to.
Also, Lonannuniel, I think tying the concept of pacifist into not hurting peoples' feelings is going to lead to you living a life as a doormat. I thought pacifists were supposed to speak out about the wrongs they see. Pacifists are against violence - if someone feels bad because you point out that they are hurting an animal, you have done nothing violent against them. It is their own durn fault for acting so badly in the first place.