I once asked MGE how she handled huge triple combinations when she could not see over the next fence and wasn’t sure where she was…she said “grab mane and kick”.
Anyway, cardinal sin is hitting the horse in the mouth and opening up too early so your butt hits their back before they finish landing. For the sake of the horse, most instructors will not move a rider up until they stop that. Usually MOST riders can understand it’s not personal and not fair to that horse.
Put that neck strap on there and she needs to learn to use it if she wants to jump.
Try lunging her no hands no stirrups. Have her work on really sitting deep in the saddle and loosening up her lower back and hips to go with the horse. Lots of deep sitting trot and full seat canter again working on loosening up that lower back and hip.
Set a pole on the ground and have her show you long, medium and short crest releases as she trots over, then move to that the canter. NO release with the body, sit down and just fold at the hip-which I suspect she cannot do-use that hip and waist area.
I have a feeling she is throwing her whole body from the heels up at a release trying to “help” the horse jump and that almost always creates getting left behind because rider cannot hold balance that way-it can also result in the face plant if they tip the other way.
Back to basics until she can better master them. Certainly would not want to move her up at all…fence gets bigger, even 2’6" and most horses will not tolerate that getting hit on the back and mouth for long. Plus she is more liable to come off due to an insecure, weak position…if they don’t buck her off.
All about developing independent hands, leg and seat before the jumps so, if this rider does not ride at least 3 days a week? Suggest the gym, particularly stretching and ab work. Pilates is excellent.