i like this letter a lot better than the one to MIL. I like Bluerain's idea about tagging him in pictures, even a wedding picture would be good. That way she finds it out without you hitting the send button. But if you prefer this method, then I think it is fair to notify her that her boyfriend is a married man. I would remove the sentence demanding that she stop seeing him. Your intent with this letter is to inform her of the facts. Making demands goes beyond that.
