They rode the horse and took the resistance as behavior instead of physical issues/restrictions?
Come on what kind of a riding school is that.. riding a horse LAME as in you ride it and then they become lame.. is a HUGE problem!
Good on you for not doing much, but honestly he shouldn't be ridden at all from the sound of it. You shouldn't have to pay while he's lame either. Especially since you did not cause the lameness.
Sorry.. just a little peeved. I hate when people ride horses till they lame. Once a girl at my ex job did so and I wasn't experienced enough to recognize it and I got yelled at (instead of the girl that DID it who was also Staff) so... yeah. It's just not fair to the horse, it's not right, and it creates more problems.
"Strength is the ability to use a muscle without tension"