According to Islamic law, it is forbidden to both bury Muslims in kafir cemeteries and kafirs in Muslim ones, and if such a thing ever takes place, it is permissible to exhume the corpse and transfer it to the appropriate graveyard.
Please pay attention to the following Islamic issue:
“It isn't permissible to bury a Muslim in a kafir cemetery, and the same goes for burying kafirs and their children in Muslim cemeteries. Of course, if the corpse of Muslims and kafirs are mixed together in a way that they are no longer distinguishable, they all have to be buried in the Muslim cemetery. Also, whenever a kafir is buried in a Muslim cemetery, or a Muslim is buried in a kafir cemetery, it is permissible to exhume the corpse, because the kafir has no respect and because since the Muslim has respect, he/she shouldn’t be with the kafirs.”
Concerning the najasah of the kafirs and them not to be respected, see questions: 2215 (website:2357) and 3008 (website:3260).
 Al-Ghayatul-Quswa fi Tarjomatil-Urwatil-Wuthqa, vol.1, pg.254; Miftahul-Karamah fi Sharh Qawa’idil-Allamah, vol.4, pg. 274; Madarikul-Urwah (lil-Ishtihardi), vol.8, pg.341.