Without knowing the facts, I'd speculate that as a typical teen, she'd let him know he wasn't her father, couldn't tell her what to do etc., maybe even let him know he was an a-hole on a regular basis. The disturbing part is that whether it was an ultimatum given to her, or a choice she made on her own, MG chose to keep her personal relationship with a man she knew over working to fix her relationship with her daughter. When that is combined with attention seeking on MG's part that irked even other KNOWN victims families, and an in my opinion frivolous lawsuit aimed at getting money, or the attention to garner donations, it raises serious questions in my mind regarding MG's motivation. The question being, did she lose her loving daughter tragically, years after giving her away, because of a potential payday, or is this really about justice for a daughter she believes was murdered? I personally don't have an opinion either way, but because money motivates a lot of odd behavior, I do have this question in my head.
I don't think it's fair to blame a child for abusive or neglectful family dynamics. Not even a teenager. The adult is the one responsible. The person most likely to abuse a child is an unrelated adult male living in the same home --i.e., a stepfather or the equivalent. Kolker's book also makes clear that this same man physically abused Shannan's sisters.