It very much depends on the place you're donating to. A lot of lesson barns don't take good care of their horses, allow beginners to use a bit before they're ready for it, etc.
I would feel very comfortable donating my horse to the non-profit that runs at the barn where I board (and that's what I've told DH to do with my horse if anything should happen to me) because I've seen them bring in many skinny horses and improve their physical and mental health. They care more about the health of the horses than making a quick profit off them.
In a roundabout way, I think it's actually a good thing that the horse in the OP colicked- otherwise they wouldn't have sent him back and he would have continued to suffer at that barn. It will take a lot longer to get him in good condition than it took for him to lose it, but hopefully the owner will be pickier about where he goes if she ends up selling/donating him again later!