We feel the situation should be taken seriously.
I am not surprised that she has denied the truth of her own e-mails. Denial is usually the first response when we are caught doing something wrong. If you approach her with a willingness to understand why she is debating using drugs or has been using drugs, as opposed to coming at her with anger, blame, and shame, you will open the door to what could become honest, meaningful, vital discussions about her drug use.
I doubt that she will continue to write or give you access to any of her drug-related e-mails. If she remains firm in her stance that these e-mails mean "nothing" and you cannot penetrate this shield, I would recommend family counseling and/or individual counseling for her. If you know the parents of these e-mail friends, you also need to decide whether or not to tell them what you have seen.