I'd tell the people she was leasing the horse from that the kid is whacked. Then I'd tell the mom to go eff herself. Then again, I am not a nice person.
This is the page I'm on. That Psycho Mommy is a bully, pure and simple, and standing up to her is the only way to go. If it's any help, this syndrome is older than the hills. My aunt encountered the Mother-Daughter Team From Hell at her barn--in the 1950's!