Have her keep track of how much she lifts each session along with a food journal. A lot of women with slight body-size issues obviously equate food with fat. After lifting weights while keeping track of diet, they will soon notice that food=strength and that's an amazing revelation for some. It has the potential to completely change her perspective of her body (as it has for many on this board already). But, worst case scenario, she will just learn proper workout protocol and become better acquainted with weights.
I wouldn't tell her this. Just have her keep track of everything and see what happens Do ask, if she has a weaker day, what she ate in the last 24 hours.