Sorry to hear about that. My daughter is a "horse person". She is married now, in her twenties. She has been into horses since she was 5 or so. She does 3 day eventing. Did it in college and still does it some now. I am not a horse person. I like horse. Love to look at them. Loved going to the events. Even love being around them.
But they are an unusual addiction that I do not understand. That is not to say anything negative about them or people who love them.
Anyway, my wife and I were the enablers. We bought her her first horse and then her second and then helped pay for the third even after she got married and moved to the country (where she is perfectly happy).
It all came to roost when her second horse got kicked in the leg in a freak accident in the pastures. My daughter was in high school then. They nursed and watched the horse for a few weeks but X-rays showed no healing. The horse had to be put down. My daughter was there for all except for the final event, administered by the vet. She helped arrange a back hoe and picked the spot. It was hard on her but she dealt with it.
Last Christmas her first horse, very old by now, got bad colic. She was too old to treat, it was Christmas eve and things were getting bad. Fortunately, her horses now stay in her father-in-law's pastures. They live right next door and her father-in-law is a very kind, very down-to-earth guy who grew up around cows and livestock all his life. He put the horse down on that night since there was no getting a vet out there and nothing the vet could do. He used a rifle as he had been required to do many times in his life. My daughter dealt well with that as well.
My daughter has bred her third horse and they now have that filly. She will train it for eventing. Her husband recently got a Marsh Tacky and they have a fourth horse that is a border. These horses seem to take up all of their time and money but it makes them both happy.
All of this is to sat that I don't necessarily "get" you horse people. But, I've been on the margins of that lifestyle and it has served my daughter and her new husband well and in that regard I can sympathize with your loss having seen them deal with it up close.