If the social worker doesn't remove the child and something happens to the child, she will be blamed by the system for not having removed the child before the thing happened but if she removes the child(even wrongly) nothing will happen because they will say it was in good faith(the mother had addictions in the past)
That seems to me like it would be a problem anywhere, I remember a case just like that recently here in MA and the outcry was very loud about how could they not remove the child from the home.