Girlfriend trying to cheat on me

[mason55]

So my girlfriend (1.5 years, we live together but she's moving to LA for the summer to start a business, I am in DC) left an email open on my computer last night. She was contacting a guy in LA about a discrete relationship. I have her email password but have never used it because I trust her. So far they're just introducing themselves.

Here's the question. I'm obviously going to say something. Do I confront her now or should I wait a day or two and read the emails they send back and forth and see where it goes?