We the leg immobilized -- as in a cast? Is the broken off piece of bone too big to remove? It appears too small to reattach with a screw. What I have seen of fractured sesamoids, if they are not able to be surgically repaired when they first happen, they only get better if they can be immobilized with the small fragment resting very closely to the bigger bone. Even then, they often do not reattach.
If the fragment cannot be removed, I see no way this horse would ever become sound enough to not have the other front leg break down. I would also recommend euthanasia if that is not a possibility.
I know a lot of people think with their hearts and not their heads, but ultimately the value of the horse and the cost of doing extraordinary things to save a horse that will have a very poor quality of life need to all be considered.