He's in Iraq, She's cheating...What should i do?

From my point of view ( I am sorry I have to say this) he does not love you but is merely using you to get back at his fiancee. He is confused, understandably so, and is hurt. Try telling him this, but also include that you are there to help and that you will defend not only him but also her. Find a counsellor for them, but on the same note do not get to involved. It might get nasty for you. Best of luck