Did you assess what’s happening at the joints above?
I’d start by looking at calf and peroneal flexibility. Have client do a static calf stretch (traditional, one leg extended back long with knee straight but not locked, foot flat, for gastrocnemius). Make sure both hips are pointing straight forward, both knees are straight forward, and both feet are straight forward and back. Correct any of those issues and see what how that feels with client – could be tight with just those corrections. Next step = ensure that the big toe on the back foot is pressed into the floor so client can feel bodyweight on it. Next progression = same stretch, but in addition to big toe down, lift pinky toe up.
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.