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?