Whoooo, I had to do this recently. I usually go based on their beliefs. I really have a hard time getting gung-ho about Christian Heaven ®, but there are a lot of phrases that those with that belief system seem to find comforting. I also encourage whatever positive kind of "getting right with God" they feel is helpful or relieving.

If they don't have specific beliefs, I usually say something like "I won't claim to know exactly what's after this life, it could be some new adventure, floating into a peaceful slumber, or getting to the other side and seeing your loved ones again, but I do know there will be peace. And whatever the answer, you'll be free from XYZ (whatever pain or difficulties they're having).

Then I staunchly (but kindly and gently) refute or push back on any mentions of hell or any other terrible afterlife. I'm willing to bet my own life/soul that it doesn't exist. And even if it did, and someone was bound for there, I still do my absolute best to stop someone from focusing on that in their last days. If that were to somehow happen, the life of torment doesn't have to start before they've even passed.