Welcome Bonnie,
Your story makes me think. If it was my mother I would feel much the same as you...abandoned. My alcoholic is my son and as I think it is not so different. Yes, I am the child of an alcoholic but my most immediate issue right now is my son.
I feel abandoned like you and I have a front row seat watching him ruin his life. After many years in recovery I simply know through experience that I cannot make a dent in what he chooses to do. It is the same with your mother.
She has had the opportunity to get help, she has listened to your concerns and still she returns to...in your words...her best friend. Booze.
There is slogan in Alanon about "Letting Go". Simply put, it means getting out of the way. Your mother is an adult and has a right to choose. You may not agree with her choices but they are still hers. You can love her and support your father but there is very little you can do to change the situation.
I love my son but I have had to get out of his way too. If he should ASK for help I would be there but right now he is not asking.
Hugs,
JT