I don't consider it a smart thing to do unless you have unlimited funds, and vets on call 24/7/365. Even then, there's no guarantee of recovery.
Nobody's certainly doing the horse
any favors by keeping it alive with its coffin bone sticking out.
Anyone running a rescue would be a fool for even considering it. Heck, even just a regular horse owner doesn't have that kind of money to waste.
Instead of spending enormous amounts of time, money and resources on just one animal who would AT BEST be pasture sound, those things would be better utilized on a number of animals who have a decent shot at living a normal, pain free life.