if she's lost she might turn up again.

She will always be in your heart and thoughts.

I'd have to turn it back to them "What do you believe?".
It wouldn't be the time to put my beliefs forward - more the time to ask them to put forth their own beliefs and discuss whatever brings them comfort.

It’s a difficult one to answer truthfully if you don’t believe that he will. I would just say that it’s not what you think that’s important, and that nobody can possibly promise that he will, even though others may do so. I would try to get him to concentrate on the wonderful times they had together, and how that love will never die, regardless of what happens in the future. Be there for him and the family in a practical way, by listening and supporting them, if they are religious, as as presume they are, there will be plenty of others praying and telling them they will meet again in heaven, no need to contradict this, just leave them to think that they may if it brings them comfort.