All the barns I've been at have required the owner of the horse to personally hand written permission for said person to ride owners horse to the BO, and/or the trainer, so that there are no he said she said arguments about horses being ridden by people who aren't supposed to be getting them out. Someone once got out my Thoroughbred, things were moved around, and she was upset next time I went out to see her, fortunately we all had individual tack sheds, so no one could use my tack, but I was never able to prove who did it. Please keep us updated, and let us know what happens. I would hope that they pay for the damage to the saddle, and the training its going to take to get your horse back up to her previous level. I would also move her as soon as possible, before something worse happens.