@donasmrs-- We can't diagnose one another with psychological disorders and addictions! Only a qualified psychiatrist can do that! You should speak to a doctor on how to approach your girlfriend about her seeing a doctor herself.
I'm not an expert or a doctor but I do have an opinion on this topic because my sister is a narcissist and alcoholic.
It is true that a narcissist can become more problematic and abusive when drinking alcohol. Alcoholism, along with other addictions are emotionally destructive for narcissists because addictions push people away. Loneliness is the worst thing that can happen to a narcissist, because most narcissist thrive on affection, care, praise and attention.
Some also thrive on negative interaction and might look for a fight if he or she feels that positive interaction is not possible. This is why alcoholic narcissists tend to be more aggressive, upset and abusive than other narcissists.