You obviously have a deep caring for your girl and that is wonderful.
Hormonal imbalances are horrible. In my experience- I was put on Zoloft for SEVERE pms about 2 years ago, which was the WORST drug I have ever taken- staying away from alcohol is going to do a LOT to reduce mood problems and depression from pms.
Perscription drugs are only so effective- they are VERY specific and often have undesirable side effects. For example, the Zoloft made my pms-related depression much better, but really severely increased my anxiety, and I started having frequent panic attacks.
This combination of things helped me the most of all:
1) it's step 1 for a reason, a good therapist whose style suits her needs, to sort out the problem properly. Like, how much of it is the PMS, how much is the drinking, is she having side effects from other drugs, and are there other factors?
2) lay off the sauce. I know it is a major social thing during and after college for a LOT of people, but you can't deny its role in depression.
3) possibly consider changing perscriptions. My new therapist was aghast that someone had perscribed me zoloft with my anxiety, and also said the birth control pill that I was on at the time was REALLY a contributor, and that I should change or stop it. Different pills have different effects on women. I know I will never take an oral contraceptive again!
Anyway that's the help I can give for what she can do to get her mood back right.
There is not much I can say about your interactions... if she is addicted to pills and alcohol, it may be best to talk to someone in her family- does she have a sibling who she listens to? This sounds actually more like her mood problems are a result of addiction than purely hormonal. Until she acknowledges that, neither of you can do anything to solve the problems.

Could it be that she is happy to accept the diagnosis of PMDD because it lets her off the hook for her actions? Think about that....