Keep it simple.
Just add a couple of movements to the warm up that will help develop the squat. Even if it's not perfect, it'll give you a chance to coach it and her a chance to practice before every work out, without making a big deal out of it (it can be frustrating putting too much focus on something you just can't grasp when all you really want to do is "tone up").
Give squatting to the step a chance and see how it goes. Most likely, she'll break at the knees, heels will lift, and weight will shift way forward possibly with some wobbling. This is where you get to test your coaching cues. Now, if she's breaking at the knees AND going up on the toes, don't say, "break at the hips and stay on your heels". Just say, "move the hips back first". Forget the feet, you'll probably find the route of her technique break down is lack of hip hinge. Fix the hips and the feet will follow. I get the hips doing what I want them to do, then I cue the feet/knees (grip the floor and rip it apart).
Even if she starts to move the hips back first, it may look like an awkward good morning with the knees caving in. And you know what, that's progress. Leave it there after 10-15 reps and get on with training. She'll be better next time. Most importantly, she probably won't be frustrated.
I used to be the worlds worst for over-coaching. Wanting everything perfect the first time. I could easily spend 2 hours on one big lift with one person (and I have done). Using the warm up as a "feeder" to the training sessions took a lot of frustration away from me.
Anyway, give that a shot. If she's REALLY bad for hip hinging, there's still options. I'll generally use a DB Sumo Deadlift (I'll explain this if you need it) and also tend to use a "dowel hip hinge" as a warm up movement, too - very difficult to do this wrong.