I think you should respond with "I don't think there's anything to talk about as long as you're still talking to OM."
Yes, you lose advantage because now she knows you know. But it doesn't matter because you need to stop snooping any ways. She will take her activities underground and keep lying - don't buy any of it.
Why do you need to meet up with her? Now the situation has changed. You are not interested in having any relationship talks as long as she's talking to OM. That's your boundary. Your self-respect and that you're not a doormat.
And then start living your life and DBing like a pro - GAL, detach, drop the rope, etc.
As East said - don't get drawn into a fight. After you text her that, she's gonna come back at you. Turn off your phone if it's too hard to deal with for the day or leave it out of sight. Don't fight her, ask for explanations, explain how you know - NOTHING!!!! Let her stew in what's she's created.
