I’m still incredibly skeptical that an adult three legged horse would not be grossly lame at the walk.
I’d also expect laminitis at some point.
I would expect that your friend’s mare would be written up as a case study by their vet for research purposes. Is there any record?
I honestly would have, too, until I knew a horse about 20 years ago who badly fractured his scapula in a nasty fall. He lived almost 10 years with a poorly healed fracture and significant nerve damage that left him essentially non-weight bearing on that leg. He never did trot again, obviously, but he could walk, gallop; and even buck. I wouldn’t have chosen to keep him alive like that, but he coped surprisingly well and never experienced laminitis.
Keeping this foal alive is poor torture in my mind, though.
I can imagine a horse being able to compensate from an injured front, rather than from a partially missing hind, without laminitis, but either handicap I would think would incur quite a risk.
Believe it or not - i saw that mare every week at our common grazing. As I said, she was TOUGH - physically and mentally. Her walk wasn’t really a walk anymore - but a sort of strange combination of a walk/trot/canter/hop and she got very smooth at it. Being a rear leg probably helped.
Angela (the owner) was very very very committed to managing this horse and providing her a good life including all of the physical therapy that you could imagine ever needed and probably some that we wouldn’t imagine. At considerably increased time spent at the stables - well, she bought her own small farm because of it or at least as her excuse to buy it.
But the mare couldn’t be on hills anymore - which she had lived on for most of her life. Perhaps she did suffer a bit mentally because of that - but there was simply nothing showing that unless we consider that the cancer was caused by stress … but the mare was a grey so who knows.
I am very clear that I would not recommend this at all if I had owned this mare. It is one of my absolute nos. NONE of my other friends would - and I doubt that Angela would do it again but that mare was special. Having experienced this - at a distance! - I can state quite clearly that it is not a good idea to keep them alive.
Albeit, a good that came out of this - my friend Angela is now a human massage therapist - and she is an awesome for human amputees.
The new mare they rescued looks bad. She has a huge hole in her chest from running through a fence. Its nasty to look at but hopefully will heal up okay.
As for that foal I think euthanasia is the right decision. Not sure how long it will take them to decide that.

Not sure how long it will take them to decide that.
I admit that I am quite cynical when it comes to the motivations of (some) humans. So here’s my take: This rescue is fundraising off of this poor 3-legged creature and until that well runs dry, or there’s enough outcry over the cruelty of it all, they’ll keep him alive.

I’m sorry, but that is the sort of birth that calls for “instant decision time” before the poor foal even takes a breath. Anything else is plain cruelty.
I don’t think you even needed to preface this with “I’m sorry”. It’s the truth.