I remember a heated debate over a video posted on the ACE website
http://www.acefitness.org/article/2555/ discussing the proper mechanics of performing a forward lunge.
Aside from performing the lunge with a smaller ROM, your client can perform an assisted lunge by holding onto the wall or a body bar for balance to focus on correct form.
I’ve elevated the front foot on a step with a client once that seemed to help her.
If lunges are still tweaking her knee than there are several other exercises that will strengthen her lower body that she can perform while the others are doing lunges.