She can technically sue you for anything.
At least here in Texas, she probably wouldn't have a leg to stand on. ANY activity with a horse, regardless of disposition, is seen as an at your own risk activity, whether it be your horse or someone else's horse.
She would have to prove gross negligence, which I doubt she would be able. There are a million reasons for a horse to buck. My horse threw me off the first day I got on her, without any previous warning or indications she would. It was due to saddle malfunction that wasn't obvious.
She may have pushed the horse in an unfamiliar situation, she may have lost her balance and grabbed ahold of the horse, startling it, could be she just rides differently, uses different cues and the horse got frustrated- no telling.
In your case, I would offer help with situation, without saying fault (as this is business, it is always good to offer help, and if goes to court will also be a plus- since that always looks good. If she refuses, not your problem) Gather witness statements, and any video of the horse working with you. Document all correspondence.
Sounds like she's just po'd (which I probably would be, but I wouldn't necessarily sue over it)