Is it obligatory to inform the Muslim fiancée and her family of her fiancé's atheism, especially with the wedding date drawing near?

If you are certain of the suitor's atheism and his persistence in it, you must inform the family of the Muslim fiancée about this. This is because the marriage of a Muslim woman to a non-Muslim is invalid by consensus. To confirm his atheism, it is necessary to understand what makes a Muslim an apostate, as not every doctrinal disagreement constitutes apostasy. Generally, if you know of a defect in the suitor's religion or character, you must advise the fiancée's family. It is obligatory to mention the suitor's flaws as an act of sincere advice, even if you are not consulted, unless the need for doing so can be met without mentioning them.