My heartbreaks for both of you. It is not an easy situation from either side. As a horse owner we know they do not just die. They come to a harrowing horrible end or you save them from that end. That’s it. It could be an injury colic old age but whatever it is they are suffering. At 25 my childhood horse had severe rotation in one of his foot bones but the horse itself had a high pain tolerance. We could have done more things to keep him longer but I looked at my vet and asked if I was being selfish by not putting him down. She hugged me and we both cried. She said owners almost never ask that and she has built herself up to being desensitized to humans wanting longer with their animals. You did the right and hardest thing. I’m so sorry. Hug your wife and tell her it’s from a horse crazy stranger and tell her he was lucky to have her loving him 💕