This posture combines hip rotation, flexion and abduction. Notice the difference in how much each student is able to bend knees and flex the hip joint while still keeping the knees mostly tracking the toes and the tailbone dropping.
For the student with a stable horseman’s pose, its not necessary or even possible to keep the knees fully tracking over the middle toes. So just have the student stay aware of staying moderate in the posture knowing that a wide open and 90 degree angle pose will never be possible.