A few good suggestions here. look at abductor/adductor strength. Also look at glute med strength on the weak side.
If--and I highly doubt everything will be about even--but IF everything is about evenly matched, have her do single leg squats for the weak leg (bulgarian split squat). Actually, you should probably have her do that anyway, and only for the weak leg.
Another teaching tool would be to use the upright of the squat cage to hang onto (with no bar at all, just bodyweight), and have her lower into a full squat position using the upright to balance her and keep her from falling back. Then have her focus very hard on using her weak leg while she can balance herself with the upright.
goblet squats could also help, perhaps. But look at the other things first. I also second looking at a hip misalignment, although that will probably need to be done by a professional/chiro. If it is obvious enough for you to see, it might be a huge problem :).
EDIT: also forgot, check into her glute/ham strength when you are doing the bodyweight squat assessment...perhaps the problem is not the quad but the fact that one glute/ham can take more weight than the other (using it to support the stretch reflex). If she is sitting back, that could be a problem unless the squat depth is high.