The only thing to watch out for is 'failure to thrive'........children need plenty of protein and I would be watching their weight and height in comparison to other children at that age. I'm sure you can find it online somewhere, but there's a graph chart that outlines the growing curve and breaks it down to percentages. Once the percentage drops below a certain level, it can reach the catagory of failure to thrive, especially if it extends over a long period of time.
And MB is right..........you cannot dictate what she does in her home any more than she can dictate what goes on in your home. The only difference would be if something within the home becomes harmful or detrimental to the child. In this case, your voice in this won't mean a thing, but the recommendation of a doctor could change it if they felt the situation warranted it. In cases like this, a third party objective opinion carries much more weight than coming directly from the parent. Just something to keep in mind over the long haul.............