You can not tell her to leave anybody it has to be her own decision in her own time.
Your telling her to leave him can only lead to problems like her resenting you, her thinking you are a control freak and her wanting nothing to do with you.
You don't really know what she feels for him or if she would ever really want to leave him
She may say she wants to leave him that does not mean she is seriously considering it or planning on it.
