Hmmmm how did nobody know she was riding your horse and get it sorted first? By any chace is she another friend of the bo?
Also there are laws stating that you can't take out another horse without the owners written consent
I'd be finding out if this is something the BO allows this girl to do. She obviously didn't think she did anything wrong or she wouldn't have told you what she did. If is normal for people to ride horses belonging to others at this barn, then I would move my horse as soon as possible. The BO should be able to put a stop to it though. I board my horse and although I don't think it's posted in the actual barn rules, it's just common sense that you don't ride, groom or mess with someone elses horses in any way. I don't see how a boarding barn could be ran any other way.
There's no legal action that can be taken, and if if there was she's th eone at fault. That horse is yours, not hers. She has no right to get on your horse without your permission. Its like trespassing or using someone's property without permission.
Plus, if the chiropractor could explain in court the hazard to the horses health that she caused with the riding then that could also be considered property damage.
There are some nasty BO's out there Speed Racer. Not everyone is ethical. I took a lesson at a barn where some very icky things went on- such as the BO using a boarders stud to breed her mares without his permission, and allowing her children to use other people's horses when their friends came over to go riding.