To clarify, we obviously don't mind if the middle seat is occupied it is just luck or not. The point is the other passenger was clearly in need of assistance and had evidently been identified at the gate if not before and was then preboarded to his own non exit seat. He was then moved by the female flight attendant to 12e the middle seat in the first exit row on a lgw based A320. Senior crew then told the passenger and flight attendant that he could not sit in 12e and moved him back to his original seat and told him he could return to 12e after take off , which he then did. I consider this a potential hazard as I understood that only able bodied passengers could occupy exit seats for any part of the flight. He occupied the seat for all the flight excepting take off and landing