A Jew buried in a Christian cemetery
Question
Shalom, My father didn’t want a funeral but he was given one anyway likely with a minister. They even put a Christian symbol on his tombstone. While my father wasn’t observant he wouldn’t have wanted this. Would it be appropriate or possible to have some sort of blessing consistent with his faith done posthumously at his grave?
Answer
Shalom,
The best you can do for him is to bury him according to Jewish law.
Like you've said, he was not interested in a Jewish funeral, but his Jewish soul is experiencing a tremendous pain while he is not buried properly.
You can pray for him and recite kaddish during the first year of mourning.
However, it is prohibited to pray next to his grave since it is marked with the idolatrous image of a cross.