There are several sects amongst the Shiah, some with somewhat mild corruptions in Aqida like the Zaydis. If the person belongs to the Twelver sect (who for example believe in the Ismah/Innocence of the twelve Imams and that most the Shahabah [Maadhallah] became apostates upon the death of the Prophet, peace and blessings upon him) then these are beliefs that render a person outside of Islam and marriage is unlawful with them. As for the other sects, who do not reject the Sahabah etc.) unless one does not first correct the belief of the person to accord with the belief of the majority of the Umma i.e. Ahl al-Sunna wal-Jama`ah, then one should avoid marrying, as even a minor aberation in belief, even if it does not take a person out of Islam, is not to be sought. Thus in view of this you should advise your friend to teach his wife the correct Aqida of Islam, and not divorce her as it would seem she is not from the categary of Shiah that have beliefs that take a person out of Islam.