From the picture she looks like she should gain a little weight. Certainly not overweight.
BMI, or any effort to use a simple formula to determine health is stupid and inaccurate.
When I was in the air force they started using a bike test measuring heart rate. It was supposed to measure how efficiently your body used oxygen. We had to take it at our assigned station. I was stationed at Cheyenne Mountain (NORAD) at an elevation of 9,564 feet. They claimed there was no difference between this elevation and taking it at sea level which is what the metrics were designed. Our significantly higher failure rate was simply due to our laziness.
The girl’s mom could stand to lose about 30#, but the girl looks to be a very healthy weight for her age.