I know he's cheating, what do I do?

Ok, my fiance and I are in a long distance relationship, which has been hard but it seemed like things were going really well for us, were getting married next summer and I was going to go live with him in a few months. But this morning I logged into his email to look for an email that had information I needed for something (he usually doesn't mind me going onto his account, because I help him send emails when he's busy) anyway, the email wasn't there so I looked in the trash and I saw a couple emails from someone named cindy, and when i clicked on them they were about getting together with him and I saw that he had sent an email to her in the first place after seeing her ad on craigslist :( he said he was horny and he wanted to hook up. I don't know if he ever did hook up with her because it looked like it may have been a scam and I think he figured that out from the looks of his last email. But what do I do? He's coming to see me next week, should I tell him I know or should I ignore it and try to be extra good to him? I really love him, I still want to marry him and he's so good to me, I never would have believed he would cheat on me if I didn't have the emails as proof. Please help me :(