Talk to your friend when s/he is sober. The sooner you can arrange this after a bad episode, the better. Your message will have more impact while your friend is hung over than it will a week later.
Restrict your comments to what you feel and what you have experienced of your friend's behavior. Express statements that cannot be disputed. Remarks like, "Everyone's disgusted with you," or, "Lily thinks you have a real problem," will probably lead to arguments about Lily's problems or who 'everyone' is. Avoid such generalizations.
Convey your concern for your friend's well-being with specific statements. "I want to talk to you because I am worried about you," or "Our friendship means a lot to me. I don't like to see what's been happening."
It is important to openly discuss the negative consequences of your friend's drinking or drug use. Use concrete examples from your list. "At the party I was left standing there while you threw up. The next day you were too hung over to write your paper. It makes me sad that these things are happening in your life."
Emphasize the difference between sober behavior that you like and drinking behavior that you dislike. "You have the most wonderful sense of humor, but when you drink it turns into cruel sarcasm and you're not funny anymore. You're mean."
Be sure to distinguish between the person and the behavior. "I think you're a great person, but the more marijuana you smoke, the less you seem to care about anything."
Encourage your friend to consult with a professional to talk about his/her substance use problem. Give them concrete resources, like the Counseling Center.