I want to forward that email thread to my W, but I am sure that is not the right approach.
Well, at least your instincts are good. Not even a difficult one.
I'm sorry you're having to continue to uncover additional deceit. I'm sure there's more, but it's best to continue a "I know all about you and SG" stance and keep it vague. When you tell her you know everything, and she doesn't know what you know and what you DON'T know, she has to assume you do know each piece. Make sense? As sukky as it is, there is power and leverage in that.
By all means, if SHE ever brings up in conversation, something like "Well, like when I went away on my girl's weekend, we . . . ." you should put your hand up in the "stop" position and say "PLEASE STOP IT. We both know you're lying to me right now. I know all about who you were with that weekend, and it's incredibly disrespectful to sit here and lie to my face. We don't allow it from our kids and I'm certainly not going to accept it from you." (and leave) . . . but to bring it up now is NOT a good idea, as you already know.
Did you see my question yesterday about your finances?
Not even a difficult one.