How much energy the impact of the slab, the pulled gear etc took up, I don't know. I DO know that the piece that caught the climber was less than 10 feet above the belay. The climber ended up at least 20 feet below the belayer. Let's say 30 feet of rope was out. That puts the theoretical fall factor OVER 1. Obviously the impact lessened that SOME, but any fall where the climber falls below the belayer is going to hit hard.
If the belayer didn't know how bad a high factor fall could be, yet took rope burn on both hands and stopped him, then she should be commended. If you don't know what you don't know, then how would you know that you are unprepared to handle a situation like that? Good thing this ended up like it did and all involved (and not) can learn from it.
i think blowing the gear and hitting the slab reduce the fall factor well below 1...