There are only three people allowed to ride my horses when I'm not there. My trainer and two of my advanced students and I have it in writing. Even then they call me before riding to make sure everything is good to go.
If something "undid" training on one of the horses by someone riding who wasn't supposed to be riding I'd be getting video of everything if I could. I have video of previous riding on all my horses and I'd get video on the after math of my horse being "untrained".
If possible can you send a friend in with a camera to a clinic in your place to catch the lady in the act. Maybe the friend can pose as someone interested in possibly boarding or taking lessons and would like to audit/sit in on a clinic at the facility. That way you might be more likely to catch the lady?