Captain Danny Ross says that the shot came from across the East River. Considering that the duel reenactment (and filming) was on Governors Island, this would be incorrect. The southern tidal waters of the East River (which is not actually a river) flow between Manhattan and Brooklyn. Therefore a shot across the river from Lower Manhattan would go across to Brooklyn, not Governors Island.
The sniper would need to fire the rifle several second BEFORE the duelers fired for the bullet to arrive on target when they fired. It can take several seconds for the bullet to travel a mile. The sniper would need to know precisely how long after the "aim" order the "fire" order would come.