It sounds to me that you weren't misleading at all. If you told her about the bucking and then she bought the horse, you've done nothing wrong.
If he's bucking alot more with her, then the problem could lie with her, if you say he only bucked once on a trail with you.
Don't stress too much over it, you did the right thing in the first place and if she decided ti buy the horse then it's her fault for not listening to you.