Whiskey's Butt

Whiskey the horse has had a very persistent infection at the base of her tail since her fall off the trail six months ago. The vet has repeatedly cleaned the wound and put her on antibiotics; both oral and injections. We don't know what else to do. This is getting VERY expensive. She is a part of the family, but this is also a business. If nobody wants to take her and take on the vet bills for a 25+ year horse, do we put her down? What a miserable dilemma.