For Atheists only
I would feel quite comfortable (at the right time) saying something like "it must be comforting to believe that you will see each other again".
There's the risk they might use it as a preaching opportunity, but personally if it made someone I loved feel good in that situation, I'd let them preach until they were blue in the face.
But thinking of it from their perspective will help you comfort in a way that has meaning to them.
It's not the time for attacking their beliefs.
All the best.