Work her off the end of a lunge line. That will let you help the rider to move her forward if need be.
Make sure she lunges well in both directions before you put someone on her back.
Unless there is good reason to do so, I personally would not use a western bit if an inexperience child is going to be riding. Many can be very harsh on a horse if not used by an experienced rider with good hands. I would start with a simple snaffle while you lunge her and see how she does with it.
If you do go with a western bit, PLEASE don't use a tom thumb - people sometimes use them thinking they are easy on the horse because the mouth piece is "broken", when in fact they can be quite cruel like a nut cracker on the horse's jaw. Start with a short shank and low port. And don't forget to have her teeth checked. Especially at 19, she could easily have problems that could cause her pain. A lot of "misbehavior" can be solved or avoided by having teeth floated.
Good luck!
Edited to add:
And I agree with Jayne. Sounds like she probably needs to start back at the beginning with some basic training.