Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
Making duaa for non-Muslim dead parents has no avail and is even prohibited by explicit Quranic injunctions.
Answering your question about making duaa to non-Muslim dead parents, Dr. Muhammad Salama, PhD, Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu), states:
Making duaa for non-Muslim dead parent has no avail and is even prohibited by explicit Quranic injunctions.
In Surat At-Tawbah we read the following two verses,
{Therefore, do not ever pray over any one of them who dies. Nor stand over the grave of such a one, for they have disbelieved in Allah and His Messenger and died while they were ungodly.} (At-Tawbah 9:84)
{It is not for the Prophet and those who believe to seek forgiveness for the idolaters- even if they are [their] close relatives- after it has become clear to them that they are the Companions of Hellfire.} (At-Tawbah 9:113)
The Quran clearly prohibits praying over and seeking forgiveness for those who die as disbelievers.
Almighty Allah knows best.