Hi, I'd like to help you with your question.
It sounds like you are on the right tract. However, I highly recommend that you try to find a neutral third party to help you with the intervention. When you do an intervention, the person you are confronting often feels attacked and therefore becomes defensive. A neutral third party can help keep the intervention on an even level and help the person feel that they are not being attacked.
When you do the intervention, keep the conversation non accusatory. Do not raise your voices and use "I" statements only. Do not bring up someone else's behavior. You do not want her to start defending someone else. This will keep her defenses up since it is easier to come to someone else's defense in such a situation.
You may want to consider working with a therapist to help you do the intervention. You are talking about doing a co dependency intervention so you would need to find a drug and alcohol counselor that is experienced in interventions. Try contacting the local community mental health center in the area you will be staying in. Or you can try on line at http://www.findatherapist.com/.
Also, you may want to pick a neutral place as well. Since you will be in a different city, this is probably easy. Do not do the intervention in your hotel room but another room in the hotel that is private. Also, if you chose a therapist to help you, you can work in their office which would be the best place since it would be a neutral for everyone.
Here is a book that may help you with the intervention. It is called Love First: A Family's Guide to Intervention by Jeff Jay, Debra Jay, and George McGovern. It is available on Amazon.com or your local library may have a copy as well.
I hope this has helped you,