I agree with others, there is just no wrong and no right answer in a situation like that, it will depend entirely on each individual person's desires, needs, and feelings.
What I would do is weight the costs/outcomes of each individual route.
What are the odds that the pony will come out of this sound and pain-free?
Can you personally afford to continue on with the expensive treatment?
(rhetorical questions, you don't need to answer them except to yourself)
Basically, whatever you decide to do, whether it's continue to fight for the pony or decide that the risks are just too high, any true horse-person would understand the reasoning behind it and support your decision.
If you do what is right by the pony, your family, and your daughter, then you made the right decision.
Good luck with her and keep us updated on her progress.