I see this saying of "Rest In Peace" for the death of a non-Muslim as one of the major fitnahs of our time: the "need" for Muslims to want to "fit in", or their weakness with respect to their own 'aqidah, or their ignorance of what Allah jalla wa 'ala teaches us in the Qur'an, or their ignorance of the Seerah of the Prophet (sallallahu 'alayhi wa sallam).
To say RIP or Rest in Peace was originally a Catholic du'a for their dead, and stems from the latin "May his soul and the souls of all the departed faithful by God's mercy rest in peace." and indicates a request to God to allow that soul peace in the afterlife. This original meaning doesn't change just because now every Tom, **** and Patel uses the term for anyone who dies. It's meaning is it's meaning because it is never used for anything outside of showing sorry over someone's death, and Muslims do the same for those souls who deserve such a du'a i.e. Believers.
The issue is that Muslims are strictly prohibited for making such a du'a for someone who dies upon kufr (Steve Jobs was a Buddhist and we judge upon the apparent, not dream about the hidden or make extravagant irrational connections between him and his Muslim father). This is a matter of consensus amongst the scholars, because Allah forbade His Prophet from doing the same as He forbade Sayyidina Ibrahim the same, and He said in the Qur'an, "And the intercession of the Intercessors will not benefit them" and He said, "Not even if you seek forgiveness for them seventy times" and He said, "Verily, those who disbelieve and die, and they are still disbelievers then the curse of Allah, the Angels and all of mankind is upon them" and He says, "Whoever takes partners alongside Allah then of a surety Allah has made Paradise impossible/impermissible for him and his abode shall be the Fire" and so many more evidences.
And any argument that "we don't intend any of that by saying RIP, we're just being polite/expressing our sadness" is just ridiculous and indeed shows that ignorance I mentioned above of what people will justify to themselves just to fit in. RIP will always mean that you wish peace for a non-Muslim when Allah jalla wa 'ala has decided otherwise. DECIDED that is (based upon their death) not something up in the air or not.
Of course, this is after their death. Whilst they are alive, we can desire and seek guidance for them and wish and request that their souls do indeed rest in peace by accepting the rights due to the One true God whilst they are still alive. And of course we can and should show sadness and regret for the loss of our loved ones or admired ones, whatever their religion. But don't lose YOURS over that loss. That's all.
Wallahu a'lam. A reminder for myself and all others.