This made me so mad I could barely sit still to finish reading the responses. I probably would have taken that chain bit from her and beat her with it. I also would have threatened to bill her for any and all chiropractor bills for riding your horse without permission, and vet bills for any new problems, laughed at her mother and retaliated with threats of my own legal action, and THEN I would have reamed the BO a new one, because I'm assuming she/he knows what's being done for your horse, and should have DESTROYED this person as soon as she touched your horse. Give them all a piece of your mind, and find a better place.
"Keep a leg on each side and your mind in the middle"