E.M.
I wouldn't worry about it... As long as this isn't new (as in she ws 30% last year and now is at 2%) she could have a family history from dad's side. My kids all show signs of my side in the underweight area and there is nothing you can do about it.
I am 5'6" and weight 98 pounds... I am VERY underweight, my kids are all underweight and my parents are both underweight. In fact my dad was drafted for the Army back in the day and he weight too little for the ARMY. They tried for 6 months to get him to gain and NOTHING worked. He couldn't even gain 5 pounds. I used to have my Jewish friends call me "the holocaust victim". Believe me if I COULD gain, I would, but please make home a safe place for her and don't make it one more place she feels judged for being "too thin".