Everyone's probably right that there isn't anything you can say to convince her that this is a really stupid idea.
But - if you want to give it a go anyway - tell her that her body is going to get the protein it needs to sustain life one way or the other. If she doesn't give it healthy and nutritious food, her body will turn to the protein stored in its organs - such as the heart! - and muscles for its daily needs. Ask her if she really wants to 'lose weight' by digesting her own internal organs or whether she'd rather lose fat by doing it the healthy way.
