I'm not defending her behavior whatsoever.
But, if your husband knows the step dad has these pills and goes to his mom saying he is in horrible pain with big ole crocodile tears running down his face, and swears that he needs them so he can (fill in the blank) work, walk, not be in pain, etc., and that pills have never been an issue, only booze, it would be hard for a naïve mother to say no.