Have to agree with everyone else here.
You stated "before our coach flipped out at her in a jump lesson, yelled at her and told her to ride her <bleeped> horse more"
So you know that her trainer has ALREADY told her this. This is an issue between her and her trainer ONLY, it does not involve you.
And believe me, she knows EXACTLY where she placed in shows, and she knows EXACTLY what the trainer has said to her. She doesn't need you to repeat it, and to be honest, the way you say it comes off as quite rude.
She knows she isn't riding perfectly, but one of two things is likely going on:
1) she doesn't care - riding might just not be as important to her as it is to you
or 2) she doesn't have the time - there could be family things going on at home, or other things in her life, that prevent her from putting the effort into her horse.
In either case, there's nothing you can do about it. As long as the horse is being cared for, it's none of your business.
Dealing with these issues is your trainer's job. That's what she has experience doing, and that's what she's paid to do. Trust her to handle it appropriately; don't try to take it in your own hands.