Praise be to Allah.
It is not permissible for this woman to marry this Ismaili man, because the Ismailis are heretics who are beyond the pale of Islam.
The scholars said concerning their madhhab (school of thought): “It is a way which outwardly is Raafidi but inwardly conceals pure kufr…”
Ibn al-Jawzi said: “What they say is a denial of the Creator and of Prophethood and of the acts of worship, and denial of the resurrection. But they do not manifest this openly at the beginning. Rather they claim that Allaah is true and that Muhammad is the Messenger of Allaah, and that the religion is true. But they say that these concepts have a hidden meaning which differs from the apparent meaning. Iblees has deceived them and has made their way attractive to them.”
Similar rulings were applied to other groups besides the Ismailis who follow bid’ah and who were deemed to be kaafirs, such as the Nusayris and the Raafidis. So it is not permitted to marry any of them, or for them to marry any of the Muslims.
It was narrated that Talhah ibn Musarrif (may Allaah have mercy on him) said: “The women of the Raafidis should not be married, because they are apostates.”
Shaykh al-Islam (Ibn Taymiyah) said in his discussion of the extreme views of the Raafidis and of the Nusayris and Ismailis concerning ‘Ali, that all of these kuffaar are worse kaafirs than the Jews and Christians. If one of them does not make an open display of that, then he is one of the munaafiqeen (hypocrites) who will be in the lowest level of Hell, and those who do make an open display of that are the worst of the kaafirs in kufr. And he said: it is not permitted to marry their women, because they are apostates and are the worst kind of apostates.
Concerning the Nusayris he said: the scholars are agreed that it is not permitted to intermarry with them, or for a man to marry his female relative to one of them, or to marry one of their women.
Mutawaatir reports from the righteous salaf indicate that it is forbidden for a Muslim woman from among Ahl al-Sunnah to marry someone from among the followers of bid’ah who has been judged to be a kaafir, and that this type of marriage is null and void.
See Mawqif Ahl al-Sunnah wa’l-Jamaa’ah min Ahl al-Ahwaa ‘wa’l-Bida’ by Dr. Ibraaheem al-Raheeli, 1/377-380
And Al-Taqreeb bayna Ahl al-Sunnah wa’l-Shee’ah, by. Dr. al-Qaffaari, 1/152
On this basis, it is not permissible for this Muslim woman to marry this man, because he is not a Muslim, even if he claims to be, as was stated above concerning their madhhab. She should not continue thinking of this haraam matter. And Allaah knows best.