If you want to say something Pass, my advice is to write it in an email where you have comfort, time, and minimal stress and can say and do what you are comfortable doing. AND have time to think carefully about what you're going to say.
And be sure to reinforce your position on the infidelity.
Good conversation needs skills, and cheating while you are "talking" doesnt' demonstrate a great deal of conversational talent to me. No, you aren't having sex while you are talking, but it shows that the foundation for conversation isnt' respected.
That is the first step to a productive talk is a safe foundation.
