Isolation is exactly the thing that initiated all of this. He used to tell her "If anyone talks trash about me, or me and you together, just tell them, 'I hope you don't mind that I tell Craig just what you said, because I'm going to...'" And the sad fact was, she did. She trusted him because she wanted to. Which, in the beginning of being enamored of someone like Craig (God knows why, but we each have our preferences) is understandable, because you want to be on the "same page" with your bf/gf. But they took this to the nth degree and it became this odd 'pact' between the two - kind of a "You're as sick as the secrets you keep" kinda deal... And boy, have those sick secrets escalated.
I also know... well, for the purposes of this board, I believe... that this is a lot about rec drugs and not about who at least one of these people would have been without them.