Honestly I'm not with anyone on it. 1. It just shouldn't have happened in the first place with the prank as it's just plain stupid.
2. Using a taser on someone is just plain cruel without real justification which I wouldn't say that is. If the prankster threatened the old man then taser the crap out of him. But for that no. Not what was deserved. Call the police. If it did happen irl. He just left a massive DNA sample on your car...