I’m a Muslim woman. My sister told me that the man she wants to marry had become Muslim when he met her. Now they’re living together and I discovered that he's not Muslim. I decided to not see her again because she's not Muslim any more. Did I do right or wrong?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

We advise you to be kind to your sister.

You should advise her in a good manner.  You have to explain to her that Islam does not agree with what she is doing, and that it causes the Curse of Allah, The Exalted, to be upon her.

Even if that man is a good Muslim, it is not permissible for her to live with him, or to stay in seclusion with him (without the presence of a Mahram), let alone a Kafir.

Bukhari and Muslim reported that Ibn Abbas (Radiya Allahu Anhuma) narrated that the Prophet (Sallallahu Alaihi wa Sallam) said: "A man should not be in seclusion with a woman except with the presence of a Mahram".

If she lets the man to take pleasure in her and fornicate with her, then this is a major sin and an enormous offense.

You have to remind her about the punishment of Allah and His severe chastisement, may be she will be deterred and frightened.

If the man embraced Islam and wanted to marry her, then he has to follow the normal procedures, i.e. to inform her guardian that he wishes to marry her, that is after he showed that he is following the Islamic jurisprudence Muslims manifest after declaring the Shahadatain.

It is not permissible to accuse her of becoming a Kafir just by that act; this act alone does not take the person out of Islam, although it is repulsive, obnoxious and extremely dangerous.

Allah knows best.