I've heard of several reports of a hoof being found out in the field and trying to identify which horse it belonged to. When this happens naturally, that hoof is sloughing off only when the hoof inside it is ready to face the the ground. The time it takes for the hoof to slough is a good thing.
No, the whole hoof doesn't come off. What you will see is a hoof wall with a bit of sole attached to it. There are 3 bones going down the fetlock and into the hoof. They are not going anywhere and the name of the game is to protect that last bone that's in the hoof. (P3) Unlike every other bone in the horse's body, P3 won't remediate if hurt, it will just destruct, so its important to follow the vet's orders and do your homework diligently when she comes home.
What the vet will do, probably is to be the least invasive as possible. The goal is to address the infection and get it draining. The infection is the real culprit here. He may also take what is loose and about to go anyway....no use walking on a broken fingernail, jamming into/hurting the new growth.
Don't worry. Your horse sounds like its in good hands. Just do diligent homework with the vet's instructions.