How do we know this marriage is a sham? Who are you to judge what their relationship is like? She cheated on him, it is awful, I wouldn't do it, but sad thing is in this country a lot of couples have adultery in them. But just because she cheated on him doesn't necessarily make their marriage a sham.
But you have no right to decide and be like: "I am making an executive decision here for you two, and it is clear to me you two are not happy together and don't deserve to be together. Therefore I am going to end your marriage by telling your husband we fucked after I voluntarily agreed to help you cheat on him".
The wife has every right to tell him what she has done, it is their life together and if she believes he needs to know she can tell him. But you telling him that does no good for your "friends" life. It only hurts him more, and essentially makes you take this fucked up ethical high ground of: "I know what is best for you, you deserve someone better," when you are the douchebag friend who helped put him in this position to begin with.
Ignorance is bliss. What happens if he comes back from being deployed, they start a family, and they both live a very happy life and she never cheats on him again? At the end of the day would you rather want to know the truth and be miserable, or not know about some things but live a very happy life? Truth be told I am not sure what I would want if you look at the options like that... But I sure as hell wouldn't want my douchebag best friend who fucked my wife to make that decision for me.