I'd drop her. I had a talk recently with my gf who was over at a friend's house and her friend was smoking. I told her that it was her choice to be there when her friend is doing it, but I don't want to know if she's there. I also told her that if she smoked once to try it, I didn't care, but if it became a habit, I'd drop her like a hot rock.
It's not that I have anything against it, I have a few friends that smoke. However, I've made it known that as long as I'm around, I'd appreciate it if they didn't smoke around me. They respect my wishes, even the ones that smoke a few times a day.
My gf was talking about how it's her friend's life and she can make the choices she wants, but I pointed out that her actions could potentially endanger my gf's future, which isn't my gf's choice, but she said, "Yes, but I accept the risk." I told her that I didn't care if she hung out with her friend, but I did care if there was smoking going on.
We've just agreed to disagree. Like I said, I have nothing against it personally, but I just don't see the appeal, it is illegal where I live, and in my opinion, there's better things to spend loads of cash on that last me longer than the average high probably would for the same amount. I don't think I get any less enjoyment out of doing things that I really like, like gaming, martial arts, and various other things, that I would if I spent the same amount of time getting high, and I'm probably more productive because of it.
Also, I'm not risking my future college plans to get stoned. Being high is temporary. A criminal record is not.