You may not ask Allah to forgive or have mercy on someone who died upon disbelief; Allah says it is not fitting for the Prophet or the believers to ask forgiveness for the polytheists, even though they be of kin, after it has become clear to them that they are companions of the Fire (Quran 9:113). The Prophet ﷺ himself said, "I sought permission to beg forgiveness for my mother, but He did not grant it to me; and I sought permission to visit her grave, and He granted it to me" — so visiting the grave is allowed and is a reminder of death, but the duʿāʾ for forgiveness is not. You should not take part in non-Islamic religious rites, though scholars have differed about simply accompanying the body or being present at the burial for the sake of family ties, so ask a trustworthy scholar about your own circumstances. Grief itself is not a sin. We judge only by what is outward and the final affair belongs to Allah, so do not declare a particular person to be in the Fire; honour them by the good you do, and be excellent to the living members of your family — Allah commands kindness and justice toward relatives who do not fight you over your religion.
Q&A · New Muslim
My non-Muslim parent or relative died — can I pray for them or attend the funeral?
Evidence — the daleel
On deen2u
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.