Accident versus negligence
I think the reality is that as an owner you are stuck, but an ethical trainer/boarder pay if there were negligence involved, even if a bad decision was made in good faith.
Examples:
My horse “got away” from me in the barn aisle of a large barn. He tore down the aisle and took some horses in cross ties with him. Thank God no horses were hurt but one horse lost a shoe (I paid the farrier bill, volunteered to!)
My sister’s horse was in its stall. Someone hung their bridle in such a way that he could chew the reins, and he did. The boarder wanted my sis to reimburse, and she refused – her horse was being a horse, and stuff happens.
A mare was mistakenly turned out in a pasture with geldings and mahem ensued. The mare got kicked. The mare’s owner paid the vet bills, but in my opinion she shouldn’t have. Negligence, poor training and supervision of barn staff.