"...I don't know whether that would be repairable or not. If not, how the hell would they fix it? A metal bar to replace the bone? Or would they have to amputate the lower leg?...."
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Moto, they will probably use an external fixator, because the damage will be too close to the ankle to employ a tibial nail.
Something like what Bloodline posted (which is an Iliziarov frame) or even a traditional external fixator.
Everything depends on whether his vessels and tissues are salvageable.