I’m giving advice as a small business owner (an author) myself.
She can talk to her friends, she can rage offline in real life, but she has to let it go (even if she’s 100% in the right) online.
It’s especially difficult given that this is a rather complicated scenario to explain (again, even though she is in the right), which can quickly turn into a game of telephone she said/she said online. You don’t want to distract from the positive associations with your brand online. You learn from the experiences, and, as others have said, don’t send two finished products to someone with the expectation one will be sent back, especially without getting payment for either.
Also, putting a customer “on blast” is off-putting for me as a customer. Even if the business is in the right, no one (especially a horse person and a trainer) likes to worry that if you buy an item from this person, and there is a conflict, that they might throw a tantrum online. A business today lives and dies by positive reviews.
If there wasn’t a contract about sending one back, I think it will be hard to fight in court for a small item (even if there was a verbal contract). Also, if the trainer has friends, it’s not a great way to secure a foothold in the industry, especially in the horse industry, to get a reputation as being sue-happy.
This is time to reevaluate business practices, and chalk up the loss as lost inventory.