I agree with Kenneth. If you can't find any structural injury e.g. ligament/muslce strain, meniscal damage, trhen i would look to increase the stability of the knee though a strength programme.

Just as a side line, have you assessed her pelvis to check that the source of pain isn't from there. My line of thinking is that following pregnancy the hormone relaxin is still in the system, whcih affects the laxity of ligaments around the pelvis to allow for birth. She could have a pelvic problem casuing altered biomechanics leading to her popliteal pain. Just a thought...