One of our French brothers embraced Islam, and died, may Allah have mercy on him. Unfortunately, his family refused to hand over his body so that he could be washed and shrouded, and the funeral prayer offered for him, and they buried him in the Christian graveyard, according to their ways.
Our question is: how should the funeral prayer be offered for him? Should we go and stand at his grave and offer the funeral prayer for him in the Christian graveyard, or should we offer a funeral prayer for him in absentia in the mosque? Please note that we are able to go to the graveyard.