Supporting someone going through the struggle of fighting addiction is not for everyone. By not exploding with anger at her, or walking out of her life you are essentially saying "you are of value even if you cant see it right now". During that moment of weakness the addict just wants to give up on themselves and be left alone with their addiction. By making your boundaries clear you are validating your own self worth, and rather than using the promise of walking away if things dont change as punishment but rather the natural consequence of this behavior, you are diffusing her ability to search for excuses to blame you for what she has done even to herself.
The reality is, it takes time to learn new healthy coping skills when someone really doesnt have them.. so yes I do think relapse in the early stages of the process probably is a near inevitability. But as long as the addict is really working on it, isnt encouraged to give up on themselves or emotionally coddled and enabled, it should become dramatically less likely to happen again.