I read your story, and I am so sorry you are having to go thru this with your daughter.
My insights come from being an addictive type of personality, to substances and relationships.

Your daughter, and the fellow AA member to whom she has become attached, probably know, as Leed stated correctly, that romantic involvment with a fellow member is considered forbidden...."13th stepping." When members are new, and vulnerable, struggling with addictions, etc., they are not ready for a relationship, according to AA. The older man should (and probably does) know this.

At one point in the past, a counselor told me bluntly, that addiction to a person can be as strong, or stronger, than any drug, and even harder to break.

I only learned to conquer and control this tendency toward obsessive relationships with a lot of hard work, some counseling (some helpful,some not), and the sobering conclusion that I'd have to do it on my own...abstain, just like with any substance. But I was/am much older.
I had to WANT to break the cycle, and I finally did reach that point.

I hope your daughter will get all the kinds of help she needs, including counseling.

Patty