Advice on how to support an alcoholic?

My best friend is an alcoholic. She went to rehab last year and was doing well for a while but in the past few months has had trouble. How can I support her? What can/should I do when I know she is drinking or about to? When/should I step in? Do I even have the right to do that? I don’t want to overstep any boundaries but I love her very much and I just want to help. For reference, we are extremely close and have been for years.