What is the father's duty towards his daughter whose husband refuses to pray and embraces the Quranist doctrine, and his daughter has been influenced by him, and what is the way to divorce her from him?

What was mentioned about your son-in-law concerning his denial of the obligatory nature of prayer with bowing and prostration, his denial of the Sunnah and his claim of adhering to the Qur’an alone, and his statement that Jews, Christians, and others will enter Paradise if they are truthful and connect with Allah – all of this is considered disbelief (kufr) and apostasy (riddah) from Islam by consensus. The Qur’an commands prayer, and the Sunnah clarifies its description. Thus, the Sunnah explains and elucidates the Qur’an. Anyone who claims sufficiency with the Qur’an contradicts the Qur’an itself. It is obligatory to separate him from his Muslim wife, because a Muslim woman is not permissible for a disbeliever. As for the verses that mentioned the virtue of those among the Jews and Christians who believed in Allah and the Last Day and did righteous deeds, what is meant are those who were monotheists before the prophethood of Prophet Muhammad, peace be upon him. However, after his prophethood, nothing but Islam is accepted. Whoever does not believe in Prophet Muhammad, peace be upon him, is among the people of the Fire.

It is obligatory to call him to the truth. If he insists on his disbelief, it becomes obligatory to separate him from his wife. It is also advisable to counsel your daughter to return to her senses and seek divorce or khul’ from him, while striving to save her from this misguidance. If you do your utmost and she persists in her misguidance, there is no sin upon you.