NEVER reveal the source of your intel, and never show your evidence. Just say "I know everything that's going on between you and _______, and it needs to stop. Immediately. It's incredibly disrespectful to me, to our marriage, and to our family. I will not live in an open marriage."
Don't do anything else for the next 48 hours. She will BADGER you to try to find out what you're going to do; just say "I haven't decided yet." Let her twist in the wind.
I'm really sorry, I know this sucks. It's like getting kicked in the gut by a mule. But you WILL survive this, and -- if you do it right -- you WILL come out stronger than you have ever been.
And, even though you can't control it and I can't guarantee it, even your marriage can come out stronger.
