The story states "they had been separated". We don't know who left who, or if there is a divorce. But. If it was her who divorced him, it would implicate her all the more since she would have had some grounds to sue for divorce. Possibly his drug driven debauchery.
What would fill in some blanks here is knowing what sort of ongoing contact, if any, existed between the mother and father. We can assume there was little to zero contact with the daughter previously or there would be no need “to build a relationship”.
I'm betting mom knew about dads sexual habits and drug use. She may not have known just how twisted he was, but that's no excuse. She should have vetted this guy before green lighting visitation.
So long he does not criticize her and the equally cold and evil goberment system, dads like this can have visitation.