"W, I just wanted to let you know that I am aware of your affair and I am disgusted by your behavior." That's it. No more needs to be said. If she denies just look her straight in the eye and say "we both know that's not true." If she persists just say "I just wanted to tell you that I know, and that's all I have to say about it".
THAT IS ALL. Do it calmly, without anger or hatred or anything in your voice. You're just informing her. DO NOT ENGAGE IN AN R TALK, which is what you're trying to do above.
