Praise be to Allah.
We advise your sister to stand by her husband in his repentance and attempt to rid himself of addiction to drugs, and not to leave him to the human wolves who would destroy him. He needs her more than ever right now.
You have to seek help from medical centers that treat addiction. There is nothing wrong with that, because this will help him to get rid of his addiction.
She should not hesitate in doing this, and she should hasten to get in touch with those who specialize in dealing with such matters. But she should always remind him to be pious and fear Allah, and remind him of death and the meeting with Allah, whilst encouraging him to give up his addiction and giving him the hope that he will recover and his repentance will be accepted by his Lord.
Please also see the answer to question no. 6540.
And Allah is the Source of strength.