You need to realise that there is nothing wrong with your friend smoking the odd joint. She is doing nobody any harm.
For you to judge them show's a serious lack of judgement on your part.
You really need to try and overcome this. The problem here is you and your narrow minded opinions.
Live a little and relax.
OP, if you and your friend are as close as you say, I would talk to her. I doubt it will offend her to know that you have a problem with her drug taking since she probably has an idea of your stance. You can probably stipulate that you don't want to have anything to do with the business and that you'd rather not hear about it either. Once you are both on the same page about it, I don't see any ethical issue with continuing your friendship.