I was going to post saying perhaps it didn't occur to her she was crossing a line by handling another persons horse, but I changed my mind. I can't think of a single person I know who would think it ok to mess with someone elses horse without the owners permission...
Anyway, OP, did you say anyting at the time about how it is inappropriate to try to load someone elses horse? Or try to train a horse in a way the owner does not approve of? Tell her why it was a dumb idea and tell her never to handle your horses again without clear consent from you and you alone. Then take the moral high ground and ask her politly if she would like to learn how to load her own horse properly.
Never know, she might be humbled enough by your rant for a moment to learn something, instead of pretending she knows it all already.
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz