First, who organized the trip? If it was someone else's idea, I would defer to that person. If it was an idea that a bunch of you came up with jointly, then you have the power to deal with this on your own.
How close are you to this friend? How would you feel if the relationship did not survive this issue?
I have a similar "friend." I feel bad for her situation more than anything else, and we used to be really good friends 30 years ago, but life took her in another direction. Now, I think she needs the connection to me more than I need the connection. She's a good person deep down, but way too many issues for me. I imagine the two of us in your situation. And, if that were the case, I'd sit with her and tell her that I care for her, we go way back, etc., but ... we are all going on this trip to have fun with no worries, and her history has us worried that if she went to Vegas she may have trouble with the alcohol, and it may cause worries and concerns for the rest of us. Bottom line, everyone's concerned about her joining the trip. I'm telling you this because I care about you and I don't want you to be in a situation like that, and I don't think you want to be in that situation either. Etc.
As with everything else in life, it's all in how you present it.
Of course, you run the risk of her getting hurt and mad and not being your friend anymore. But, if it's said with love and caring, then you cannot fault yourself. She has been through treatment many times, and she has to know that eventually this behavior is going to sever relationships with people who do not behave like she does. Who knows, it could end up being the catalyst to recovery.
Good luck.
Louise
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"You don't really ever have to fall. But kissing the ground is good because you learn you're not going to die if it happens."
-- Jacquie "Alice B. Toeclips" Phelan, former U.S. national champion cyclist