Wow. That's odd. My pony is prone to foundering and we have never had to put him down. Are you sure that is your only option? He heals in a week with our help. We feed him extra non-foundering vitamins and water down a spot by his water for him to soak his feet in. He always ends up fine. We just have to make sure he is not laying down because that can lead to colicing. Im not saying your vet and farrier are weong, they have examined the mare and they have both came to the same conclusion so maybe your mare is different. If you have any questions I will be glad to help! Just message me!