Well, squeezing the stomach muscles takes pressure off of the back, because they are connected. She could also be landing awkwardly. Is her FHS good? Correct landing position? If so, then the problem could be strength, which I think is unlikely. Female anatomy is different from male anatomy in the hips and back, so it might just be the way her body is set. If so, there's not much you can do.
If the pain continues and her coaches say she is landing correctly, take her to a doctor. She could have scoliosis or some other back issue.
I've seen her DD's videos and she looks perfect, but I'm going to throw this out there anyway! My DD was occasionally, as she put it "twinging" her back on her FHS. We got a new elite level coach who took one look at her and said she was slightly rotating on her landing. Nothing that I could personally notice. All it took was working on that landing for one session and she's had no back pains since. Of course, now there's the knee pain, ankle pain, etc. to deal with!