I lost most of my baby weight in the first month too. I fully attribute it to breastfeeding, because I surely was not dieting, and stroller walks were the only form of exercise I was doing. It doesn't mean that my (or her) body is in its pre-baby form, it just means she can wear jeans and a t-shirt without a visible belly.
This argument seems like a lose-lose situation. Give the girl a break.