This is a tricky one, people always tell you to approach this issue sensitively. Bring up the topic gently and tell her not only how it is affecting her but affecting you (causing you to worry etc).
If you can try and take her down to one of those temporary homless shelters or rehab clinics, the stories they tell you can scare anyone out of drinking.
Don't go in to the issue aggressively, because she will react in the same way and things can get very heated.
One thing you could try asking her is the CAGE questionnaire (google it), ask her the questions and note down what score she gets.