She is looking for recommendations of an aid that will help her horse be more consistent and also have a lower head set. (Correct me if I'm wrong.)
Unfortunately, there are no devices that will teach that. Some may produce a false head set for you, but like you predicted, it does not last after the aid comes off. Also, these can be uncomfortable for the horse and can make them have other problems under saddle.
I think your solution is consistent training. I'm sorry to say I can't help you with that part. Training is not my expertise.
"The wise man thinks he knows nothing.
The fool thinks he knows everything."