It's only because he's quacking at her and she believes him. If he hasn't got his s**t in one sock yet, then it ain't going to happen with her. Don't forget - only the addict can change themselves, no-one else can do it for them.
You've had a lucky escape and have got the rest of your life in front of you to make of what you will.
If it helps, when I got together with my A b/f, I was in the same situation as this other woman (b/f had just split with his wife and I didn't realise the full extent of his problems). I thought we were so happy together, but it turns out that it was all fake. 2 1/2 years later, it's all going pear shaped.
Keep your chin up.
M