It is not permissible for a Muslim woman to remain with her non Muslim husband. She must leave him immediately and not have any intimate relationship or any kind of relationship with him.
If she doesn’t remarry and after a while he reverts to Islam, they can go back to each other without a contract providing she didn’t remarry and the choice is hers.
This sister must be separated from this man until he becomes Muslim. She has the choice to remarry another Muslim if she wishes after she gets her monthly period.