As an alcoholic, you cannot help someone who doesn't want help. It's a disease that grabs ahold of your mind and tells you to do things you actively don't want to do. It's hard to explain how you can just NOT want to be drunk today but then do it anyways. How you can know you're hurting the people around you and still do it anyways. Both of my parents were violent alcoholics, but as I've grown and dealt with the issues of addiction myself, I've learned to have empathy for them.
Whatever you do, don't marry this person unless you're 100% sure you can deal with this disease. It may get better but it will definitely get worse for a time. There is no "former alcoholic". There are only alcoholics who choose not to take the first drink.
I hate to say something so harsh but it's the truth. I'm glad my girlfriend has endured me but I'm trying. There was a time I was downing 2 pints of bacardi gold every day, waking up and filling my coffee cup with it and drinking all day, sometimes passing out, waking up and drinking some more. I've cut back but I made.the decision myself. My girlfriend definitely pushed me in that direction and I'm not trying to take credit away from her trying, but everyday I make that decision myself.
You can push your fiance, but they have to make that decision. Everyday.