How to gain weight the healthy way
I would refer her to the Paleo Magazine community. They give excellent nutritional advice and it is extremely valid. They also address such issues as weight(fat)loss and proper weight gain(muscle gain).
The beauty of it is that if you feed someone one real wholesome food, they will not go wrong. Their body will autoregulate itself to whatever weight it is suppose to be and also whatever healthy body composition is meant to be in.
Alex