I've been thinking about this situation and what you can do. What could you ask her to do that would show she's back on track again? She is not ready to come home. She was not ready to leave rehab. The best thing she could do right now is go back to rehab. I don't know if you're financially able to offer her that, but if you are, that's what I would do.
I would explain to her that she cannot come home because the very first thing she did when she got out of rehab was to lie to you and to go see a boy who is still using. If she wants your support, she must go back to residential rehab.