Augustus, one thing that you can use to your advantage when you gather intel, and/or even even you have the CREDIBLE THREAT of having good intel on a wayward spouse:
They don't know what you know.
And they don't know what you DON'T know.
Think about that for a minute, and consider how it can be used to your advantage. A simple statement like "Look, I know all about you and ________ , and it needs to stop right now," and then when they ask you how you know, and WHAT you know (and they will press and press and PRESS for both of these!), you simply say:
"I'm not going to tell you that, but you have a decision to make. I will not live in an open marriage."
OK, so assume you've only seen 20% of the communication that's gone on between them -- and only text messages. There are also, no doubt, cellphone calls, meetings/hook-ups, e-mails, maybe gifts exchanged, etc. How does she know that you don't know some of THESE, too???
A: She doesn't.
Once you confront them, and use just SOME of what you know, without revealing ALL that you know or even HOW you know it, then they have to assume the whole 100% (or at least some other, horrible parts) are now known.
