I think you have to realize that mothers have feelings too. Whether or not we admit it. Whether we understand it or not. Parents can get angry and emotional about their children being deaf. It doesn't matter if their family have deaf family members like Sound and Fury Mari's parents are deaf, but she still choose to implant her deaf twin son. She has her feelings.
I don't think that her son's behavior has anything to do with his CI. I think the son is misbehaving because the mother doesn't discipline her son when he needs it. He picks up his behavior from her, too.
There's really nothing much you can do, but do not feel intimidated to be yourself and sign. Maybe she will relax when she realizes her son is doing well. Try to be positive and supportive to her like comment her on how good he is doing. There's no point in criticizing her choice in raising her deaf son.
