How might your legs move if your knees were ball-and-socket joints? My knees would move forward and backward
My legs would swing in all directions
My legs would bend and flex.
My knees would pivot but not bend.

Your legs would swing in all directions but would not be strong enough to hold up your body

Explanation:

Hope this information helps

tell their parents and make sure she gets from a counselor or therapist. be supportive and be there for her. be a kind and good friend. spend time with her to make her feel like she's not alone. i'm not a professional but i really hope they stays strong.