I agree with Kevin. I have seen horses recover from some really big holes before, but NONE of them had bone showing. There is no way to close that hole, and the risk of sepsis is extreme. She might look like she's getting better at first, but I'd bet that's one of those things where it just won't ever close, no matter what.
I understand how hard it is to put a horse down, believe me. But I just don't see any way for this to heal. Even if that wound were to miraculously close, she will never be comfortable again, forget feeling good. Unless there's a vet on here that looks at that and thinks I'm wrong, I would definitely put her down.
Learning never stops