Unless it’s some kind of prepaid package at some lesson mill type operation and you want a refund, non owners are free to just not show up. If they are trying to be nice, they might say thanks, I’m moving on but they won’t be missed. Truth is once or twice a week non leasing lesson takers simply aren’t that important to them. Even if it’s a good program, they make far more off regular leasers and owners financially committed to using their services year round.
The issue here, for me, is this particular, individual horse has ditched at least two riders repeatedly putting one into an oxer and the ER. Riders overstate their skills, horses spook, s*it happens, part of the deal. But if it happens REPEATEDLY with the same horse pulling the same stunts with different riders??? Trainers supposed to be a professional paid for their expertise, it should NOT be happening. That might require being blunt about suitable mounts they charge novices to ride as opposed to saying " I told them, they didn’t listen. Too bad, so sad about the rod and screws in her leg".
Thats not a good program. That said, horses cost and non owning riders need to accept they might need to pick up some more hours at work to find more suitable mounts in better programs.