Was she that small before she started bfeeding?
When I'm bfeeding my babies, I am MUCH smaller than I would be otherwise, like 15 lbs smaller. I don't watch what I eat AT ALL, and the lbs were still coming off and leveled out at around 6 months postpartum. Now, at 16 months pp, I am inching my way back up to my pre-pregnancy weight and trying not to eat third helpings. :)
And when I'm not prego or bfeeding, my BMI is 21ish.