If someone wants to save her, and give her the best life possible, even if it's just for a time, then what is it to you? I say go for it. No one ever said life was easy for anybody, but sometimes it's worth it.
In addition to what bludejavu said, if this is a rescue/non-profit? They have a fiduciary duty to put their mission and their donors first. That means looking at donations and deciding what is the most responsible use for them--do you pour lots of money into a single horse with a poor prognosis, or do your save five horses with better long-term medical prospects?
If it's a single private person with their own money who wants to spend and spend to keep an animal alive no matter what the cost to them or the animal, it's their money to burn. If they're soliciting donations? They have a responsibility to put them towards something with a better long-term return. "Elderly show hunter needing a flatwork-only home ASAP", "Sound plain bay OTTB who needs an off-track home", "Grade pony who toted the kids around the trailer park in a nylon halter who's wider than he is tall and needs to go now" are not sexy, shock-value pictures, but there are lot more of those type of horses needing help, and they're all a lot more likely to find permanent or long-term homes to get them off a rescue's books. Especially now, there just aren't a lot of owners with the money and space to devote to a high-maintenance pasture puff.
Poor little filly. It is truly awful that her condition was left untreated for 3 years. I find it hard to believe that, even if the surgery is successful, she won't have many painful complications down the road. If I were to make the decision, she would have a beautiful day of goodies and love and attention and be peacefully laid to rest at the end of it.
I was torn about this when I first saw it. Usually, I agree that these cases should just be humanely put down to end their misery. But here is a video of the horse bucking and leaping about while waiting for breakfast. It doesn't appear to be in any pain. I really don't think a horse in pain has this kind of reaction to anything.
That said, now they are planning on using prosthetics, rather than going ahead with the surgery. Honestly, I think if this were my horse and I was compelled to "give her a chance" I think I would go ahead with the surgery. The vet isn't some backwoods yahoo, and the price for the surgery seemed pretty reasonable.