Wow. WOW. wow. I'd be PO'd, to say the least, AND I'd be showing up with a trailer, a bill for the saddle repair and a video camera. It might also be helpful to have a pre-written contract drawn up where the rider and BO acknowledge that they owe you x-amount for the past use of your horse. I don't think you can really prove the "untraining"... however, you can add that in as part of the cost of the so-called stolen back-usage (or whatever you want to call it). As a BO, I have permission from many people at our farm to ride thier horses at my leisure, but even still - if I have the time or the need for whatever reason to use someone else's horse (even with previously given "general, whenever you want" permission), I call and ask for the specific time and purpose that I plan to utilize the horse. It is just common courtesy and respect.
UPDATE ASAP, please :( I hope you get there and you are all worked up over nothing, and the woman just liked to shampoo manes and tails and your horse was the lucky recipient that day? Although it doesn't sound very likely... Good luck.