I would also suggest looking at her lunge form from a frontal view. If she pronates in the foot her knee may be caving inward causing problems. I like to cue “pressing through the heel” of the front foot to encourage more stability. Another exercise you could try is squats with a stability ball behind the back (full range or half). I feel that the client can get the 90 degree bend that they get in the lunge. If she can do this with no tweak, you can challenge her with a single leg ball squat as well.
Hope you two can find a solution!