What a scary situation!
I think it is a good idea to work on additional stopping cues. For example, I've trained my horse to slow down to a stop when I make a deep, loud exale. Also, teaching the horse to respond perfectly to body cues and eventually riding bridleless (I suggest to do this only in the arena at first) helps in situations when the bridle is suddenly unusable. Also, training a horse to stop as soon as the rider falls might be a good idea. It has helped me in a couple of situations while riding, also - in trails.
Also, the calmer and relaxed the rider is, the more likely the horse is to stop soon.