If your friend does not want to stop then you are pretty much powerless to sway her behavior. Have an honest and frank discussion with your friend about his/her marijuana use. If you haven’t done so, let her know how you feel about her drug using behavior and how it impacts your friendship. Before you have this conversation with this person try and think about boundaries that you may want to set in your relationship. Do you carry on with the friendship and not bring up the subject? Do you want to limit your friendship to those times your friend is not using? Or do you want to break off all contact? The one thing you can’t do is take on this problem as your own and try to fix it. You can offer your friendship and support but it is really not healthy to go much beyond that.
Being a true friend sometimes involves telling the person something they don’t what to hear and accepting that even your good intentions may not have the desired effect. In the end you have to make decisions that are best for you and realize you have no control over the choices other’s make when it comes to substance use.