That's the advice he's looking for, so what you said is fine. You're right, she knows he's looking at emails. Duh. Doesn't mean she's doing anything wrong. Maybe she just not sure and wants an out and his reading of the emails gives her the chance eventually to say he's suffocating her. Maybe she really wants to trust him and is testing him. Who knows. Doesn't mean though that she's screwing around or doing anything wrong. Maybe, she just doesn't want to get really involved with someone who invades privacy without permission. Guys who beat women usually do that. On the other hand, maybe they're both 18, where these kinds of things matter and you have to ask others what to do.
And guys who are insecure about their position with the woman in their lives.
Stop reading the emails Coltrane. And stop asking about them and the dude, or you will drive a wedge btween the two of you, and possibly even driving her towards the dude. Then next you'll be saying is "see I knew you were seeing him"
If the table full of food, why complain. Enjoy it, until it runs out.