Since she says she is willing to do drug tests, tell her you would like to see a 'hair test' for drugs, and WATCH her reaction.
She can't 'fake' that one, unless she suddenly shaves her head and pubis area bald, lol
That would give you a real good idea what is going on.
In addition I agree, she has found a therapist that 'enables' her. I really feel sorry for her right now. You as a RA and I as an RA know that no one, and I mean no one, could help us until we were ready to help ourselves. I also know that my best support system were those that were also in recovery like me.
Sounds to me like you have a pretty good 'handle' on you, and will get through this, again with your own support system. It also sounds to me like she is just not yet ready to find recovery.
J M H O
Love and hugs,