That's a sad situation and I'm sorry it turned out like that. First, it's not uncommon for once you actually go from everything is awesome, we are so in love butterfly in stomach to day in and day out living together, to have real life kind of cause annoyances. You don't notice things when you are newly dating that you do when you move in with someone. My husband and I were together for three years before we married and lived together for the first time. Even after all that time of knowing each other, when he moved into my place . . . I was like "who is this person leaving his junk on my kitchen counter and socks on the floor?" Annoyed. And I had things that annoyed him too. So, that's normal. Bickering all the time though is never healthy.
As to why she went back to the ex. Not sure. Possibilities are that they were only not together because of his addiction and still had feelings for one another. Which I'm sure stings. And his new status means he may work on the addiction. They also are bonded because they share a child together. And lastly, the fact is that many who are with someone with an addiction are also not emotionally healthy. Codependency and all of that. Example, someone with an alcoholic parent may have hated their childhood, but they often repeat the pattern and wind up with someone who has an addiction when they choose a partner as an adult. It's a 'familiar' place for them emotionally and subconsciously are drawn to it. She may feel that she NEEDS to be there to help him get clean or the shock of this makes her want to be involved. I'm just not sure and you may never fully know.
All you can really focus on is that this HAS happened and you are going to have to move on. Stay really busy and know that time DOES heal. good luck