Sorry to hear that but try to not get too upset, hard as that may be. First of all, the lady might fancy herself a "classical dressage rider" but who knows how trained she is or how much she knows. I've met a lot of so-called dressage riders who are pretty useless and drive the horse down onto the forehand. Even if she is a reasonable rider, she might not have had the right style to get the best of your mare. I imagine, as she sent you an email saying that your horses training was not "up to par," that she might be one of those people who thinks her
way is the only
right way and rides the way she rides, regardless of what she's sitting on. My horse carries herself in a reasonably correct frame, uses her hind end, can be super light and flexible etc these days, but she can also go like an absolute donkey if the rider approaches it in a particularly tactless way. If she had fingers, she'd be giving certain ones to those types of people. As should you.
It also sounds like due to her conformation, your mare isn't going to be a dressage wonder and probably struggles to sit on her hind end. She probably wants a rider who is sympathetic to that. Just imagine, how she went when you got her versus how she goes now. If she is better, albeit not perfect, you and your mom should feel proud of yourselves. Not everyone is in the business of training GP horses!
Lastly, anyone who had the chutzpah to send me an email saying my horse's training was not "up to par" with their "traditional" riding style would be told *exactly* where they could shove that email. I certainly wouldn't want to lease a horse to someone like that.
Even if the horse's training isn't perfect and the horse needs more engagement or self-carriage or whatever, that doesn't mean the person who sends that email isn't being a jerk.
I have no doubt that you have made huge progress with your mare.