Liability is a huge deal. I know that you probably haven't had too much experience with that sort of thing, but all it takes is one person to do something stupid, and sue her for it, even if its not her fault and she loses everything. I watched a new girl taking a lesson once at a barn I worked at, she deliberately took a dive off the horse, claimed a concussion, and tried to sue the BO for something ridiculous, like reckless endangerment. She lost, because thankfully she had signed a release, and there were witnesses, myself included, that could testify that she had fallen off on purpose, and that even if she HAD fallen off accidentally, that it was within the normal risks involved with riding, not because the horse was a known danger. But under other circumstances, she very easily could have won that suit and put the BO out of business.
I would be willing to bet that if she had some girl putting the horses up that night yours got left out, that it wasn't on purpose. Its very likely the girl got distracted and missed or forgot your horse. It sucks, but it happens. If it becomes a more common problem, then it definitely needs to be addressed though.
"Keep a leg on each side and your mind in the middle"