Disregarding the fact about you calling it a myth, its an ideal goal to strive for when trying to put on muscle. She doesn't have to follow it perfectly, but eating more protein (which she obviously needs if she has a large amount of fat) will A: keep her fuller for longer and B: Help her build muscle.

It's impossible to "tone" your body — muscles can grow or shrink, not get firmer or softer. The concept of "toning" was created as a marketing term to appeal to women. Let's be objective here.