i posted this movement a while back called a "g-row". it was money for holding my back extension in the squat: (see 7:57 mark) http://www.youtube.com/watch?v=d4e5cuyqjqw
I add a pause at the mid and high points and emphasize the squeeze/extension throughout. it doesn't need to be too heavy or more than one quality set before she squats. also, on the descent make sure she is very controlled and is thinking about having her chest up. people tend to lose their back in reversing the direction of the weight, so lower the speed/momentum until the strength and technique is there to do it at a natural pace.
let me know if this helps!