Matt,
I worked inner-city for many years trying to help folks get jobs... I hate drugs! I've seen them destroy some incredibly talented and super personality people... they either can't or won't give up the buzz. Same with alcohol.
I feel your pain. It is a razor's edge we walk in Christian witness versus becoming the dreaded "enabler" with folks struggling with addiction.
There are better sources of advice out there as to how to befriend someone in this situation, but I am under the impression this is the arena where the concept or term "Tough Love" comes in.
The more people do things to escape, the more things they seem to do that they regret which build more reasons to escape. :(
Your likely wise to approach promises and such with a healthy skepticism, only because there may be a need for your approval. Watch that one...that is where the enabler comes in.
Help her to realize that God accepts her as she is and you will continue to care for her, but you will not stand beside her until she deals with her issues...behind, not in front, not beside... if that isn't too much semantic hoop jumping.
Folks at her level, I've found, cannot and will not get out of the mess they're in on their own. They need professional help. Shame. God can do it but not without total surrender and that seems to be the fly in the ointment of those who struggle with addiction: They've surrendered to the wrong issues.
Prayers for your discernement, God's direction and peace in your life, strength to persevere with this friend and His wisdom to know where to draw the line and how.
Prayers for your friend to come better to know Christ, His loving Grace and find healing for her demons.