Ethical question:
Ultimately only your own gut can tell you what to do here, and it sounds like you kind of already know what to do, as evidenced by your statement, "that's just not cool with me."
For the sake of examining this situation, let's repalce "smoking dope" (which is, for better or worse, illegal), with "drinking beer" (which isn't illegal). On average, a beer or two at the end of the day is no big deal for most people--good way to unwind, whatever. However if someone were to be drinking beer "almost constantly" as you say about your friend in her post, I'd say most people would have a problem with that. Regardless of whether dope should or shouldn't be illegal, and regardless of the differences between what alcohol does to a person and what dope does to a person, both affect one's judgment and perception.
There is a *significant* difference between having a smoke (or a beer) from time to time, or even daily, after work, and smoking (or drinking) it "almost constantly." If, as you say, this friend of yours is also coping with mental health issues for which she is on medication, then her drug use probably has less to do with the little zing we all get from the occasional intake of a substance (coffee, alcohol, THC), and more to do with a coping mechanism for a state of affairs she may feel powerless against.
In my view, ANY job in which one is responsible for other people--regardless of the degree--requires sound judgment and keen perception. In terms of "social status," a nurse obviously enjoys more "status" than, say, a carnival ride operator; however I would hope we would all be equally concerned over a carnival ride operator doing his/her job under the influence as we would a nurse. So the "she only works with end-stage patients" argument is moot. She needs to have all her faculties present and alert, whether she is inserting IVs or changing bedpans.
If she has mental health issues, and she works with end-stage patients, I can't say I blame her for wanting to escape with a smoke, and often. When you've made it your life's mission to heal and help those who are hurting, and the patients in your care always die no matter what you do for them, well, hell, I'd have issues and want to smoke too. It sounds to me like she needs more intervention than just the meds--hopefully she is getting counseling or is engaging in some sort of other support group to help her address her work-related stresses and her personal mental health issues.
If I were you, I'd make my concern over her drug use known, from a standpoint of compassion. I would let her know that you feel it is not helping her, or her patients, to be in this state, and that you're worried about her. She needs to feel that she is not alone, and also that she will not be judged; but also that you're serious. There is no getting around the gravity of her situation. I would tell her that I'm so worried that I'm willing to do the hard thing and let people (i.e. her supervisors) know she needs help, if it comes to it. However you also understand how important her job is to her, and you understand she probably got into it for some very honorable and admirable reasons (wanting to help people, etc.). Let her know you want to help her rediscover that, but step one is to take control of the things that are negatively affecting her, so she can grow into the spectacular nurse she wants to be. Let her also know that you will be there to support her through that (if indeed you feel you have it in you to do so), but that you will not support the drug use or the over-reliance on meds. She is in a state of learned helplessness and you will not allow her to stay there. If she refuses, let her know that your conscience will not let you sit by and allow her to treat patients while under the influence. And then be willing to follow through with that if necessary.
Remember to put you and she on the same side of the battlefield, addressing the behavior; rather than on opposite sides of the battlefield, addressing one another.She may hate you for life, but no matter how close you are to someone, your chances of losing touch with them are infinitely greater than your chances of losing touch with yourself. Your Self is the only person you ever have to answer to day in and day out, and as long as you can live with that, you're good. Chances are, she'll hate you for a little while, and then realize that you really helped her out. We cherish more the friends who can tell us to our face that we look fat in a certain pair of jeans, than the friends who will smile to our faces and let us go out on the town looking like a sausage.
Hope this helps. Good luck to you!