Aw, that's a really sticky situation :-/ I'll be hoping for the best for her... but ya, that's just something there's not really much you can do with.
I'd continue to talk to the owner and convince her that selling the horse might be the best option--she's doing nothing with the horse, just counting up bills, and there's a heart underneath every horse that someone will be able to touch. What I mean is, for a reasonable price someone out there must want a nice project horse or something... the owner probably wouldn't want to do this as it would be trespassing on her pride but a rescue isn't out of the question, sometimes they're the only places that will take mustangs.
It's a dangerous business, Frodo, going out of your door," he used to say. "You step into the Road, and if you don't keep your feet, there is no telling where you might be swept off to."