I have two teenage D's (17 and 14), and I understand how difficult this is for you.
The positive is that she has a friend she can talk to. Sneaking out, not great, but you didn't mention any substances they were caught with. This could have been much worse.
One big thing I want to remind you of, this could be happening regardless of what happened between your H and you. It doesn't matter if it's probably a factor or not. Why? The minute it becomes about the separation/reconciliation, blame is placed. Even if your D says this is why she is behaving differently. Help her with better choices and if she's willing to see a C, great.
I have one D that went into crisis mode (self destruction), she see's a C once a week now. It's helping.
I leave it to the C to talk to her about H and me and how my D feels about all of this b/c in my case my D shut down and wouldn't talk to me about it and would hardly talk to H at all. As appropriate the C invites me or H in and we discuss things with them.
As much as you can reassure them that none of this is their fault and that you love them.
I use books and movies to bring up topics that can be tough to talk about. We dicuss themes in relation to the characters, seems to be easier for my D's to make it about somebody else.
It's a tough process. My other D won't say anything about it...period. In some ways this is scarier b/c I see her as feeling inadequate. That's the worst.
Be open if they do talk and use reflective listening as much as possible.
