[What a Student of Knowledge Should Do If Compelled to Attend the Condolences of One Who Abandoned Prayer]

Answer: There is no harm in offering condolences and attending their gatherings—even if the deceased was sinful. What is forbidden is supplicating for the good of the Hereafter for sinners whom Allah has judged to be people of the Fire, because of His saying, Exalted is He: “It is not for the Prophet and those who have believed to ask forgiveness for the polytheists—even if they were relatives—after it has become clear to them that they are companions of the Blaze. And the request of Abraham for forgiveness for his father was only because of a promise he had made to him; but when it became clear to him that he was an enemy to Allah, he disassociated himself from him …” [al-Tawbah:113–114].
Thus the student of knowledge should attend condolence gatherings, while avoiding supplicating for the sinful deceased. If he is pressed to recite al-Fātiḥah “for the deceased,” he must intend by his heart “the deceased among the believers,” or say: “for the soul of the deceased—if Allah wills.” This last phrasing is better, for it places a condition upon those present.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1