Maybe validate and empathize and appreciate his apology... and that's it? (I.e. don't enter a R discussion, don't spew or say "this was your choice blah blah blah". Respond in a kind but detached way, I think. I'm not sure I would respond to "what to do" about D6 either. .. (he hasn't really asked you anyway).
And he hasn't said, "I am ready to work on our R".
Keep focusing on YOU and D6. Stay off the rollercoaster. She needs a stable parent--and right now that is YOU. Hang in there.
