She might actually be a size 6 in some company's sizes : I have run across size 0 pants  so big I was able to put them on and off without unzipping or unbuttoning the top button (they were Abercrombie & Fitch camoflauge cargo pants, just gorgeous, bought used online...Wound up having to donate them to Sally Ann :-(  ) She certainly would not be able to wear my high school era size 6s, which were made back when to wear size 6, you need a waist no larger than 25" and hips no larger than 35", and size charts were honest then : Go above these measurements even a little, and the garments  were uncomfortably tight, and it was into size 8 Fat Day clothes for the day.  Nowadays, size 6s are big enough that they'd be at least a size 10 if not a 12 or larger, back when I graduated HS (1983).