If they decided to get help, then I'd stay with them and help them get help and be supportive. But if they showed no signs of getting help and were constantly doing it, I'd end any relationship with them. I kicked my brother out of my apartment when he wouldn't get help for his pot addiction, and I broke off communication with a childhood friend for a time until he got clean from his crack addiction. Associating or remaining close to someone dragging themselves down is never a smart idea. If they want help, help them, but if they are going down, don't get dragged down with em.
I was around cocaine for a number of years when I was younger, my father did it. I've seen up close and personal what kind of negative effects it had on him and all the stress it caused our family. And for that reason, I would never get involved with someone who was dependent on illegal narcotics.