I'm not much for giving advice, but I can tell you need to get this to some kind of resolution NOW. First, I would not let her know that you can see her email. Does she know that you know she has seen him? If so I would try to get a situation where the 3 of you are together, then you should be able to subtly make him aware that you are together and with her. Then you might check her email after the event and see what the tone of there conversation is. There is no problem with men or women having relationships with others of the opposite sex, as long as all there conversations and actions are things they would still do if you where there with them.
