BlueBelle has summed the situation up very well, I want to add that your daughter has no need to feel guilty. It is not her fault that her fiancé drinks too much.
The person responsible for this sad state of affairs is him. It takes time for an alcoholic to realise that he needs professional help, but unless he does so, your daughter needs to be very clear about the risks of a relationship.
If she is prepared to live with or marry him, she needs to keep firm control of her own money, make sure there never is any form of alcohol in their home and be prepared to turn friends, either his or hers, away, if they turn up bringing drink in any form. She also needs to realise that even if he is sincerely willing to stop drinking there will be relapses - many of them, which she will need to deal with.
This can be done, but it is never easy - I know, as I have been there and done that. I honestly would not wish the same for anyone, even although I do love my husband, as I am sure she loves her fiancé.
Another point is, does she want children? Our problem started late in life, so there were not children to consider, only and adult son. Frankly, children deserve two sober parents.