Just make sure to make it about YOU (a boundary), instead of about HER (controlling).
"I cannot feel safe in a marriage where my wife is in constant contact with a man who is not her husband."
If (when!) she flips out, just say "I'm sorry you feel that way. This is a boundary of personal integrity for me, and I thought you had a right to know. What you do about it is entirely up to you, but I will no longer tolerate your deceit and your disrespect."
btw, one of the great things about the cheating mind is that it is usually paranoid, and they DON'T know what you know (or what you don't know). If you want to have a little fun with this (and there's nothing wrong with that; it helps you keep your sanity), just look at her and say "Oh, I know all about you and _______ , and it needs to stop. It's incredibly disrespectful to me, to our marriage, and to our family."
When she asks you what you know, say "EVERYTHING."
When she asks you HOW you know, REFUSE TO GIVE UP YOUR INTEL. ("I'm not going to tell you that, so please stop asking.")
You might also want to start by asking her "Is there anything you'd like to tell me?" -- give her a chance to "come clean."
Finally, do NONE of this in ANGER. Speak calmly, matter-of-fact. Do not raise your voice. If she raises hers, ask her to "please calm down." Refuse to continue the conversation if she screams at you, hits you, or in any way is rude or disrespectful and she won't stop it.
