THEN REIN-AIDS!! they have a bit of elastic in them and saves a horse's mouth from a hard handed rider. Actually most horses could benefit from the use of these reins or inserts.
http://www.rein-aid.com/ WELL WORTH THE PRICE!!!
Another vote here for the Tory jumping hackamore. Most horses go quite well in it.
Have you told her she hangs? Is she aware of it? I would maybe explain she should "train" him to stop with as little pressure as possible and explain how her seat and abs factor in. No bit or hackamore or even a halter will fix this, unfortunately.
I know it's not what you were asking, but why not focus on why she is hanging on reins?
Can you spend some lessons on teaching her better balance? Is it a physical strength/balance issue, do you think? There are off-horse exercises she can do to improve that. Or is it a body awareness issue, as in she's not realizing what she's doing?
Can you put her on a lunge line for a few sessions?
And I hate to say it, but maybe change what horse she rides? If nothing else, for a really green horse, the rides he gets now sets the foundation for the rest of his riding career. Is it worth it to have to undo a dead mouth or any other bad habits?
The OP is not the rider's teacher. Rider is a back yard owner who has made it clear s/he is not interested in taking lessons. Rider owns the horse in question. I got the impression that OP may be able to suggest a bit but that's about all. Very sad situation, I feel for the horse.
Nothing is gentle with a rider hanging on the reins.
Those of you telling the OP how to teach ....well, I hope you ride better than you read :)
Hey, I wanted to say thanks to the poster for the rein-aids info.
I am a good rider and have a solid seat and steady hands but I have been retraining a 20 year old who is very hesitant about contact.
I experimented today by using my lunging side reins as reins (just attached the elastic end to his bit and held the web part) and he responded really well. I think just that little stretch and softness reassured him.
I am definitely going to buy either the attachment or the elasti-reins.