A drug is a drug, and alcohol is the granddaddy of them all.
It doesn't matter what form of a drug one uses it will always take a person back to the drug of their choice.
It's her previous drug use that's "'calling" her and don't believe that she can handle the alcohol. (see above) Encourage her to go to a support group for substance abuse...like NA or AA. It doesn't matter what drug she has abused. The important thing is for her to get support from others who have been in recovery and know the struggles she faces. You can attend "open" meetings with her to give her your support.