No, No! No! No! No! It is absolutely not your DD's 'fault' that her boy friend is drinking.
This game of emotional blackmail, where one partner blames the other for their drinking, drug taking, gambling, abuse, violence. Is just that - emotional blackmail.
She is not responsible in anyway for his decisions to drink heavily. She was with him when he was drinking, that is, she left him because he was drinking heavily. What sort of chop logic, says he will stop when she returns.
If he really wanted to prove he had reformed his ways he would have stopped drinking, when she left him to show her that he had the strength of character and that she meant so much to him that he was determined to reform his ways.
When she left him because of his drinking, him saying he will stop when she comes back, is the surest sign she can possibly have that he will do no such thing and she will be caught in an endless saga of him drinking, her walking out, him saying he will stop if she returns, and the saga will repeat itself, and repeat itself, and repeat itself.
Whatever happens never ever succumb to emotional blackmail. Blackmail is a nasty power game, no matter how it is done.