As Adrian noted, definitely get your daughter to a doctor/PT to get her checked out. It sounds like she has extremely hypermobile joints--this may, ultimately be the underlying cause of her hip popping out, but it's still good to go to a doctor to rule out other causes. If it is hypermobility, then it could be congenital, or could be from her own doing--opening/stretching joints/ligaments to hypermobile ranges (you see this in dancers/gymnasts). She could do exercises to strengthen the muscles around the hip joint, as well as exercises to strengthen the core, but if she is suffering from hypermobility, you can't 'tighten' the ligaments--only strengthen the muscles around the joint. Also, have her pay very careful attention around the time she's on her period, as she will have elastin floating around in her body, making her joints/ligaments even more loosey-goosey. Good luck!