I believe you should back away. Number one, not your circus and not your monkeys. Should a friend or close acquaintance mention wanting the try the horse, I would relate the entire story, including that the owner stated they were not allowing trial rides and stated they were sending the horse for training. It sounds like it might be a behavior that just surfaced, and she is trying to get to the bottom of it. There is absolutely no way you can police this. Nor should you. Be happy you didn’t go and try the horse out and get injured, but beyond that this is not something that should be addressed by you.
Honestly if I got a letter from a potential buyer, who hadn’t even come out to the see the horse, I would be extremely off put and well, offended. You haven’t laid eyes on the horse nor have you seen the horse ridden under saddle. Everything else is hearsay, and if you have a video, its still not your business. I’m not writing that to be mean, just I would find it extremely out of line if someone who had not even come to try my horse out, wrote me a letter about misrepresentation on a horse they had never even seen in person.