The suggestion to use a helper I saw more as something to help the OP, the human, rather than to train the horse.
The girl needs to train her horse, yes, but she needs to learn how to do that. So, if a human steps in and works WITH her, showing her how to do it, this could work. Of course, the helper can show how to do things right by modeling the behavior, without the girl herself actually touching the rope or horse or hose.
But, it's also possible for a helper or trainer to come in , stand next to the pupil and help them hold a rope or literally grab their arm and show them how to maneuver a whip or a rope, or how to walk a nice small circle or whatever.
I have worked with a trainer that will do this. I may be working in the round pen, or lunging and she wants to show me some certain thing that I just can't seem to get by listening to verbal explanations. so, she stepped in, kind of grabbed my shoulders and "walked" me around. this was to help ME, so that I could learn to eventually help the horse.