Office of Grand Ayatollah Khamenei (may Allah grant him long life):

Insofar as a woman is married to her husband – as in the given example – it is not permissible to marry her. In fact, if a woman has some differences with her husband who is not willing to divorce his wife and it is unbearable or extremely difficult for the woman to continue living with him in such a case she can turn to one of the representatives of a qualified Mujtahid. He will examine their problem and if he finds out that the woman cannot live with her husband, he will, keeping in view all Islamic and legal conditions, conduct Islamic divorce. After the divorce is conducted and the woman has completed Iddah (waiting period), it is then permissible to marry her.

Office of Grand Ayatollah Sistani (may Allah grant him long life):

It is not permissible to marry her, if she has not been granted Islamic divorce.