If you are at all thinking you may want to save this filly for breeding, give this vet a call before you euthanize: https://m.facebook.com/story.php?story_fbid=812392422190297&id=119581104804769&_rdr
That foal had fractured and displaced the femoral neck and fractured the pelvic side of the hip (acetabulum) as well. He was non-weight bearing prior to surgery and walking nearly sound immediately after surgery. I’ve known of two cases this was done with and both were very successful.
I’m very sorry to hear about your special foal! Best wishes to you and the mare!