Thank you all for the wonderful advice. I am one of the four that organizing the trip, so unfortunately it's up to me and another girl who's also friends with her to tell her no. We had a long discussion today about how we're going to handle it. The truth is a really hard thing to say sometimes, and it's going to hurt her, but I think you are all right, it's just what has to be said. It can have two effects that I see. 1. She realizes she really has a problem and it motivates her to change. 2. It drives her further into misery and drinking. I'm seriously worried that the second one might happen.
I"ll discuss it more with the mutual friend, and then the two of us will have to send down with her, I guess. It's really hard, and I don't want to do it.
Thank you all for your advice, I think I knew what I had to do, just needed some reassurance.