By far one of the most difficult questions to answer is what do you say to someone who doesn't yet believe in Christ, but who also has a loved one who died without Him, and asks about them? The way I see it, there is only one thing more devastating than knowing you've lost someone to Hell: Joining them.
Harsh though it seems, this is actually the only answer I know how to give. If your loved one is doomed, the absolute last thing they would want is for you to be doomed too. Like the rich man in Luke 16. We see him, knowing he is never going to be free, begging for someone to go to his brothers and tell them how to avoid his fate. Anyone who is dead outside of Christ and has a loved one still on the earth would ask the same. Therefore, it is better for you, and especially for them, if you would give them the comfort of knowing you won't share their fate.
I really wish I had a better answer than this. In fact, if any of my readers could give me one, I would appreciate it, because I have actually encountered a situation where a lost soul asked me about their lost loved one. I was not even courageous enough to give this answer. If anyone has a better one, I'd love to hear it.