The website of Grand Ayatollah Sayyid Ali Sistani’s office recently has published an istifta’ regarding marriage of a Muslim man to a non-Muslim women:
What follows is the Question and its answer:
Question: Is it permissible for a Muslim man to marry a non-Muslim woman who is still married to a non-Muslim man? Is there an ‘idda period for her when she separates from her non-Muslim husband? What is the period of that ‘idda? Is it permissible to have sexual relations with her during the time when she is in the ‘idda from her non-Muslim husband? If she embraces Islam, how long will her ‘idda be, if she intends to marry a Muslim man?
Answer: It is not permissible to marry her while she is married to a non-Muslim in a marriage which is recognized by them because she is a married woman. It is permissible to marry her temporarily after her divorce and after the completion of the ‘idda from her non-Muslim husband. (The period of her ‘idda is not different from the ‘idda of a Muslim woman.) Therefore, it is not permissible before the completion of the ‘idda.
If she becomes a Muslim after having had sexual relations with her non-Muslim husband and the husband has not embraced Islam, it is precuationarily obligatory for a Muslim not to marry her until after the completion of her ‘idda. But if she became a Muslim without having ever established sexual relations with her non-Muslim husband, then their marriage will be annulled immediately and there is no ‘idda in such a case.