Welcome to SR, and I am sorry for your situation that brought you here. I imagine the great posts above are not what you really want to hear. But they are spot-on, nonetheless.
For someone with a daughter who will be dating someday, what advice would you give her if she came to you in the same situation? Because remember, she is watching you and looking for guidance.
One slip...it happens in recovery. But is this really a slip? He lost his job over it; I find it a bit hard to believe he "used just once" and it happened to be the day (or a few days) before a work drug test. It could happen, sure, but the thing is, now do you believe his words?
There is something you can do right now. Nothing. And then wait and see. Doing nothing is doing something, too. But while you wait and see what happens next, pay close attention to what he is doing, not what he is saying.