No, I don't think she has a case.
And the friend sounds like she's a straw short of a bale.
Stop answering the phone when the friend calls.
It's unfortunate that the horse no longer suits her; but if you disclosed everything you knew, their was no intent to defraud. It is just horses acting like horses.
I think this lady's recourse is to send it for some professional training or sell it, or both, in that order.
This is not to say she can't sue you, she can. Which will be a nuisance and a pain for you. I just don't think she'll prevail. A friend of mine was in a similiar situation. Nice horse, needed an intermediate rider, that was fully disclosed, ample opportunities for buyer to try the horse, buyer got the horse home, didn't keep it in work, gets bucked off when it's fresh, wants the seller to take the horse back, threatens to sue. Friend did have to hire a lawyer to respond for her; and it was eventually dropped.
Stop talking to them and maybe they'll come to their senses. Good luck!