Does she take the empty magazine out and put a fresh one in? Or is she trying to lower the slide with an empty magazine in place? If the latter, that's the root of the problem.
She needs to work with an empty gun to begin with, to get the physical motions down. That means no magazine in the gun at all.
Make sure she starts with her arms bent and the gun inside her "pickle jar zone." (That's the spot between your pecs where you hold a jar of pickles to open a stubborn lid, with both your elbows bent and the stubborn jar right up against your chest.) A lot of people don't instinctively understand that their best arm strength happens inside the pickle jar zone.
From that position, starting with both arms bent, she should simply punch the gun straight forward. Have her practice that motion without touching the slide a few times. If she has the physical action down, she will finish with her right arm (the gun arm) completely straight and her left hand (still in an almost-fist) touching her shoulder. It is a complete punch with her right arm, all the way to full extension.
After she has that punching action down, have her grab the slide with her left hand while she punches with her right. Don't have her touch any of the gun's other controls, just the slide. Tell her to hold the slide to the rear while she punches the gun forward.
Make sure she understands that she will need to let the slide slip through her left hand's fingers when it has reached its rearward limit. (Sometimes, people just don't realize that they'll eventually need to let go!) Also, make sure she knows that the slide will only move 1/8th of an inch or so, and that she may not feel it moving at all, but she will still need to let go of it. If both hands travel forward with the gun, the slide won't move. She has to let the slide go through her fingers to get it back into battery.
Also a personal note: I have always had a tough time dropping a slide using the mag release. I can do it, but it's very much easier to shove the gun forward and let go of the slide than it is to summon the finger strength to move that obnoxious little lever when it's under tension.
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