Move your horses.
You say that this woman is ignorant and abusive, and that your BO is clueless.
Sounds like a hell of a place to board, and I sure as heck would be pulling up stakes and moving on.
You can't fix stupid, nor is it your place to 'lecture' this woman on her lack of proper horse care.
I see the problem as being the BO's, not yours. If the BO will do nothing, then all you're doing is frustrating yourself.
Where do all these clueless, ignorant BOs come from? This is the first BB I've come across where people continually rant about how dumb their BOs are, and how much better things would run if THEY were in charge.
Seriously, if it's that painful to you, find another barn.