Quite frankly, I wonder how much witnessing what happened to out of him? I couldn't imagine seeing something like that and it not take a huge emotional toll it if you, and that then some physical toll on you.
Reading what he wrote, I reckon if it happened in the middle of no where he would have certainly stopped. I doubt there was anything he could do. You see accidents in cycling races, Motorsport etc etc and others keep on going. But myself, I have no doubt that accident had a profound effect on his day, whether he knew it or not.