Like I said before. This is between the horse owner and the barn owner.
I don’t care if she makes tons of money or is upside down. The agreement needs to be between the horse owner and the barn owner.
I can totally see some situations where it is a strong plus for the horse owner for their horse to be used in lessons so they do not expect to be paid for it. I can also see why it makes sense for someone to get reimbursed for their horse being used in lessons. Both make sense if that is what the horse owner is OK with.
Neither situation makes the trainer evil or the horse owner evil.
This horse owner needs to sit down with their barn owner and discuss the situation. Figure out what works for them.
It does not matter what the OP thinks is best. It does not matter what we think is best. This horse owner and this barn owner need to talk.