Have your mechanic try welding a piece of metal bar to the caliper bolt, and use the metal bar's leverage to remove. Maybe a small sledge hammer to help it start turning a bit. Had the same situation and this worked for me. Save that hub carrier money and spend it on something nice on the car
He recons he can get it out but it will take the threads in the hub with it.
New hubs from breaker are only £30 each so its not too bad