He shouldn't have deceived her and her son for all of that time.

SPOILERS.

Once he found out who Frankie and her son was to him, and when he started hanging out with them is when he should have told her who he really was, actually he should have told her that night at the AA meeting as soon as he found out she was his half-sister, when he went outside to ask her for a light would have been the perfect time to tell her.

Their entire friendship was a lie, he was pretending to be someone else, he wasn't real with her, Frankie had no idea that she was befriending her half brother who she would later on have sexual feelings for.