Read and re-read what iridehorses said.
It's entirely possible the owner's callousness and rudeness stemmed from having to put the horse down and that's her safety valve because it's something she really didn't want to do.
She could have had her own illness from which she may either be laid up for weeks and weeks at a time or not recover at all, and therefore wanted to know the horse is safe.
I speak sort of from that arena. My horses look as healthy as --- well --- horses but they are not.
Two have serious metabolic issues along with some old injuries that now beget arthritis. They "appear" healthy because I spend a lot of money keeping them that way and I hand mix everything these horses eat.
Which, by the way, I would have my hands around the OP's throat for feeding my horses over the fence - especially the two with metabolic issues. Even my neighbor's two young sons won't do that and I've never said a word to them.
IF something were to happen to debilitate me in a way that I could no longer get to the barn, all four of my horses will be humanely laid to rest in their own pasture.
They range from 16 yrs to 26 yrs and each of them has some sort of issue that merits them being special needs, even though they don't appear that way to the public.
Noone wants to take on a horse like that, over the long term, even though on the short term they think they would because I am evil for wanting to keep them safe.
Like it or not, my vet would support this. Like or not - it's life and get over it. They would be safe from the idiots who only think they know the right way to care for these horses who are very trail rideable but no longer for more than anything at a Metro Park.
One isn't rideable unless you weigh 60 lbs or less but he canters around and picks the herd members lower than him.
My friend gave her cushings pony away to someone who stood on their knowledgeable laurels, convincing my friend they knew how to take care of this horse. He was dead in less than six months because they really didn't know as much as they thought they did. It should not have happened.
Hopefully, I've made my point