The horse is a 17.1 hand TB with attitude. It decides what speed it wants to go. The rider lacks Confidence and lunges first to work up courage to ride the uncontrolled canter???
Have many of you ever stood beside a 17.1 hand horse. It's sheer size is intimidating.
I still don't agree that this lady should not bite up the horse more to her advantage. What are curb bits and their mechanical advantage usefull for anyway if not to give advantage to the rider???
I'll bet that 50% of the riders on this forum ride in curbs, alot of novices included.
The reining horses, that are so well trained ride in curbs much harsher then the majority of rider use and yet their horses are so well trained and they have loads of experience.
No a timid lady on a big big horse with attitude needs more then a snaffle, even adding a running martingale double the effective use of the snaffle if required.
Can you imagine that horse on a windy day, getting excited over something and deciding to run??? I would want more then a simple snaffle and I bet the mouthpiece on that snaffle would be big and thick to save the mouth

I also feel it is a mismatch of horse to rider. A non confident lady and a big big horse. You get far more enjoyment from a horse if the horse is better proportioned to the rider. The rider should also feel in total control of EVERY situation, no matter what comes up.
You should never feel fear when riding.