Praise be to Allaah.
If there is no graveyard for the Muslims, then when a Muslim dies he cannot be buried in the kaafir graveyard. Rather a place should be sought for him in the desert, where he should be buried and then the land leveled over him so that it will not be dug up. If it is possible to bring him to a city where there is a Muslim graveyard without going to too much trouble or expense, then that is preferable. With regard to washing a dead Muslim in a place where kaafirs are also washed, that is OK, if it is too difficult to find another place.
And Allaah is the Source of strength. May Allaah bless our Prophet Muhamamd and his family and companions, and grant them peace.