Secondly, I suggest taking D18 for a long car ride or to get something to eat, just the two of you and have a very direct conversation. In that conversation, tell her that you respect her perhaps wanting to keep some feelings or info private, but make it abundantly clear to her that she does not need to protect you from anything, that you will always be there for her, and that she can always come to you for anything. Sadly yes, that needs to be stated clearly.
As angry as you are, do not say anything that could be construed as parental alienation. Also make it clear that your w is still your daughter's mother, regardless of her current behavior.
Sadly, DW, now is probably the time to remind yourself that your relationship with your kids is yours and w's relationship with the kids is between them. Don't allow your justifiable anger to intrude on those relationships.
The beauty of DBing is you can take all the time YOU need to make a decision. And, this "friend" has shown you exactly who she is as a person. You're going to be learning a lot about the people in your life as the months unfold.
Continue to focus on keeping your side of the street clean, and take the time away to really think about what you want and how you want to live your life so you'll be ready to implement something that truly works for you and your kids when the time comes.
M 20+ T25+ S ~15.5 (BD) BD 4/6/15 D 12/23/16
"Someone I loved once gave me A box full of darkness. It took me years to understand, That this too, was a gift." ~ Mary Oliver
