Totally agree. I am NOT A LAWYER. I have no idea what country you may even be in.....so laws may differ, but at least here in the US-you have photo documentation, there are other boarders as witnesses, and frankly, as stated, it would seem the BO has already breeched whatever contract there may be by not providing a safe environment for the horses.
I would have had my horses out of there yesterday. Their safety is not worth it. Let the BO try and sue you if he wants. I would think chances are slim he will even try, and if you have the documentation, he shouldn't have a leg to stand. On.
Just FYI-if anyone ever threw a rock at my horses-I would most likely throw one at them!