Nancy’s calf stretch is a great idea. I would add some myofascial release first, though, and your NASM CPT manual should be of assistance since knees moving inward is often accompanied by foot (did you mean) pronation – arches collapsing? You should find some recommendations for that very condition with myofascial release for calves and inner thigh, stretches of calves and hamstrings and strengthening of the hip abductors which are often found weak in that scenario. The single leg squat assessment should have shown some deviations as well.
I would also expect to find an anterior pelvic tilt. If it is not obvious in the assessment, have the client stand against a wall with heels, buttocks, shoulder blades and head touching the wall. There should not be enough room between the lower back and the wall to slide a hand through. In that case I would add some of the core stabilization exercises first according to the OPT model before moving up the ladder and doing power moves like jumping jacks.