Thanks, Lani. I guess I can conclude that losing muscle mass is not a common experience. My friend used to drink a protein shake every day. Now she stopped all animal products and the protein shake. Since she does not eat a lot of calories every day, I assume she might not be getting enough protein, so maybe that may have caused muscle loss (if, indeed, she is correct)? She claims this happened in the first couple of weeks of the diet, though, and I don't know enough about this to react to her. I've been trying to help. She says because of the muscle loss, she feels she needs to go back to eating at lease some meat. I've had her experience on my mind, so when I heard Campbell say this might be a "good thing," I was hoping this was something others are aware of. If I get more info on the situation, I'll communicate it!
Protein consumption by itself does not cause us to 'gain' or 'lose' muscle. Insufficient calories can cause muscle wasting, which is a different matter.
Our culture has this misguided idea that more protein = more muscle. It it were so, we could all just lay around pounding down the aminos and our muscles would start to grow like well-watered grass. It doesn't work that way. This kind of thinking is cultivated by the meat, dairy, egg, and food supplement industries. It can be a thinking entrenchment initially hard to overcome until we get this idea.
It is the workload on the muscles, together with sufficient calories including carbohydrate and proteins that, combined with your genetic potential and rest, build more muscle. If your friend is undereating and not working out, then there is going to be muscle loss most likely.
In the meantime, I'm going to see what I can find out for you about the quote - what program was it at which you heard it?