You do not need to (nor SHOULD you, ever!) reveal the source of your intel in order to lay a "no deceit" boundary. All you need to do is put your hand up in the "stop" position and cut her off, saying "Please STOP IT. We BOTH know you're lying to me right now, and it's incredibly disrespectful. I know all about you and ______, so please stop lying to me about him."
And remove yourself from the conversation.
If she presses you for what you know, or what you THINK you know, just say "pretty much all of it," and do NOT reveal the source of your intel.
There is power in them not knowing what it is that you know or don't know, as she will have to assume you know the worst parts. Especially if your actions are of a man who discovered the worst parts.
