Or trainer move to another barn where they charge the students directly. On privately owned horses.
Barns that run like this - collect all money, decide what to give trainer - typically loose the better trainers. It makes no sense for a trainer to stay if they are good enough to build a program.
It is the trainers who can’t do better elsewhere that will stay. It is up to the barn, based on what they offer and what their market will bear, how they want to offer their services.
And, using the barn’s lesson horses gives the trainer less negotiability. The barn can’t expand their program beyond what their lesson horses can do. So they don’t have much incentive to give more to the trainer.
It would be better for the trainer to collect their fees from the student and pay the barn a flat usage fee for each student-lesson taught. Add something for use of lesson horse. 5 students in group lesson = so much $ for each of the 5. And so on. But if the barn doesn’t want to do that, they just get someone else who will accept their terms. Because there are only so many lesson horses, and each can only do so much.
What is the standard lesson fee in your area for what the trainer is teaching, based on trainer’s credentials? Trainer collects that and pays $15 per ride or something for facility use, plus $ for use of horse— whatever is the usual usage fee for a facility with what that one has to offer in the area (groomed arena; jumps; cross country jump field; etc.) And capabilities- rideability of horse.
But that is usually for teaching riders on their own horse who really want to improve, and aren’t just putting in the time.
It may be time for the trainer to consider carefully what they want for the future regardless of barn , and the best way to achieve that. And then consider if this barn still fits where they are now.