Bonnie's mother should have just never told her the truth
I get that honesty is important for many people, but I think in this case, it would have been better for Shirley to just keep the truth to herself. It wasn't going to do any good for anyone, her letting Bonnie know that she had basically chosen some guy over her own child and let her child stay bouncing around in the foster care system because she cared more about her chance to be with some loser.
She really should have just kept it to telling Bonnie she was too much of a mess to take care of her and thought she was doing the right thing by staying away.