Okay, first of all, there's no such thing as "almost" separated. You either are or you aren't. And even if you are separated, you are still married, so, technically, you cheated on your husband. With a guy who's cheating on his girlfriend. And now you want to rat out your lover to his girlfriend "so she can find someone better"? Yeah, right. Where was that concern for her well-being when you were shagging her boyfriend? No, your motive is purely revenge.
What a complete and utter train wreck you've gotten yourself into, lady. Here's my advice: walk away. No, run away. From her, from him, from all of it. Don't tell anyone anything. It's over. Be done with them. Then fix your marriage or get divorced, but stop cheating, because this is the kind of TruTV hillbilly bullshit that ensues when you decide it's okay to step out on your spouse because you're "almost separated."
Thanks for the question.