Requiescat in pace. Let him/her rest in peace. (Latin)
I see absolutely no objection in saying this: It shows a smidge of respect to someone who has died but whom one does not know from a bar of soap, which offers the same level of impersonal but still intended respect somewhat akin to our actions when we buy a poppy.............
But is not so unutterably cringe-making as the out pouring of condolences one occasionally sees.
While we're on the subject of Latin remember the other old favourite which occasionally gets forgotten here:
De mortuis nihil nisi bonum.
Cusco