Oh my! I’m sorry you’re going through this and I hope your horse makes a complete recovery.
I went through a similar situation where two of my animals died, and my horse got sick, because of the negligence of one person who supposedly cleaned her facility after a contagious disease but failed to say anything to people who picked up their animals from her farm a month later.
I paid the bills. It sucked.
People talk and reputations are made or broken from irresponsible behavior like this. I would be surprised if the reputation of the boarder, the vet, the trainer, and the barn, didn’t go down after this.