If she takes photos to the humane assoc. She is not hurting anyone, but it would help to educate her if they decided that it was not abuse, and if they do decide it is benign then no action will be taken toward the horses owner and everything is right with the world. If they decide it is then she just helped the horses, no lose situation.
If everyone minded their own business how would anyone who is suffering be helped? It is you two who should be ashamed of yourselves by telling her to mind her own business, shame on you. She is not comfortable with what she sees, so she either needs to be educated about it or decide for herself that no one is being harmed.