I dont know what to do

How fortunate that she has no children. She has no reason to blame herself and if she moves on now she will be able to make a decent life for herself, perhaps find someone else and start a family eventually. If she was my daughter I would suggest this to her and say I would support her through the breakup. Then I would back out and let her decide, never bringing the subject up again. Not interfering is not the same as parenting. You can guide her but in the end the decision is her's and either way she is going to need you. I hope she makes the wise move.

I agree that there is a culture of drinking amongst 30 somethings. A lot of the drinking is because of the way they socialise. They go to places where it is usual to drink and they drink. It is difficult to cut back when your friends and colleagues are drinking. Is there something your DD and her partner enjoy that might offer them an opportunity to socialise away from drink? Sometimes something like that will help with cutting back.

They're incompatible. He is never going to stop doing what he wants and she shouldn't try to make him. I'm sure when she met him he was the same so maybe it's time for her to walk away.

Get out now while she is still young. I wasted 20 years with an abusive alcoholic which has had a lasting effect on my children and me. She will not change him, an addictive personality is always there.

May I just ask and this really isn't meant to be contentious but you said in your original post "she enjoys a drink or two" is there a chance that your daughter has a drink problem too but just a bit more controlled than himself and they are in a way feeding of each other
Just a thought and not meant to be mean to you

If your daughter has her own circle of friends that is great but if a majority of her social contact is joint friends then it might be a good idea to join a meet-up group. These groups exist all over the country and are for people of all ages. It is amazing how many people move areas mainly due to work and know no one. If you google meet up group and the area she is something should come up. People suggest an outing - maybe to the cinema or a meal or perhaps just a quick coffee somewhere. DD set one up herself when she moved 250 miles away on her own and within a couple of months there were 100 members. She has now needed to move again and has joined another group already going.
I feel for your DD as I had an OH who developed a drink problem. It spoiled my life and those of our DC. I wish her strength whatever she decides to do - I only wish I had left years before.

You don't mention if he drinks during the week. If not then he isn't really an alcoholic and there must be something else going on. Is his work particularly stressful? Is he concerned about their relationship (I know that sounds contradictory as his behaviour seems to be damaging it)?
I agree you must continue to support your daughter and stay neutral ie try not to criticise him or you might alienate both of them.

Joanna you can be an alcoholic and binge drink at weekends many functioning alcoholics don't drink every day

I recognise this. My first husband was exactly the same and died early due to the complications of alcoholism. I divorced him but to this day regret I did not do it sooner.

tanith writes a good post and the OP could do worse than read that herself.

The thing that hit me in the eyes from these post was the OP stating

my daughter wonders if there are issues in their relationship which trigger the behaviour and that makes her feel guilty.

This is absolutely so not true and I hope the daughter is verbalising this as part of reconciling herself to the fact that she needs to walk away from this relationship NOW. I just hope she does not waste years of her life trying to change, negotiate or accept this behaviour. She is worth far more and deserves better. I wish her well.

BlueBelle to be fair the OP said her daughter enjoys a couple of drinks now and again. And if she has to drive, she likely is not drinking when he is.

I have two three glasses of wine spread over a week with food, that's it. I consider that 'now and again'.

SunnySusie, my heart goes out to you- it's so hard watching the ones you love struggle, especially when they feel helpless.

PLEASE get her (or even you could) to contact AlAnon- my friend said they saved her life. Her husband was an alcoholic and they gave her strategies and encouragement- they do not judge but give much needed support. Please encourage her to at least phone them they are not there for the alcoholic, but families and friends of...

My very best wishes to you both.