The mother says that she saw the father shake the baby during a crying fit and did nothing to stop him. This mother knew what was happening and did nothing to prevent or stop it.
Drug addiction is a disease.
It is also a choice. People are not responsible for their disease, however they are responsible for their recovery. Drugs and baby killers don't belong in the same sentence. The neighbor is either very naive or uninformed. Regardless of the condition of the baby, and all they care about, once again, is the actions of the police officers arresting these people.