I usually avoid appeals to emotion because when used as arguments they're a type of logical fallacy; they have no bearing on the facts. If there's a Heaven, for example, my disbelief and incredulity cannot destroy it. If there isn't, though, all the belief and desire in the world won't create it.
That said, I'll do what I can.
If there's no afterlife, the most important thing to remember is that your friend is not suffering. Not only is he no longer in any pain, he does not feel deprived of anything - sight, touch, stimulation, emotion, companionship. It's tempting to think of death without an afterlife as a bleak, desolate place, but it doesn't matter because there's no one there.
The combination of body and mind that literally comprised your friend is gone, and can never be recovered or reconstituted. In a great many other ways, however, your friend is far from gone. He left an enormous legacy which will last until the end of time (if any). People who experience this legacy will regularly think of it as him. They will say he "lives on" through it.
First and most obvious are memories of him, including yours. You won't always remember everything about him, but there are plenty of things you will remember for the rest of your life. His death negates none of this. Memories of him will be written down and become stories, and be passed along even after those who remember him have also died. Even if nobody ever reads the stories, they will always be around somewhere, ready to bring him back once more.
Second is the body of works he wrought in life, the legacy he actually worked to create. If he had children, almost the entirety of his genetic code survives in them, together with a great store of memories and stories. His career has had a permanent effect on the economy and the business world, whether large or small. If he was creative, his art or craftsmanship can be preserved. If you know what he wanted to be remembered for, by emphasising these things you can actually positively influence future concepts of who he was.
Finally there are the effects he had without even meaning to, or knowing: the air that was displaced by his body, the nutrients and water that passed through him, the electronic signals spreading into space from the calls he made on his cell phone...there's an endless list. It may be that nobody ever traces a future occurrence (a breeze, a flower, a burst of static in a far-off space probe) back to the existence of your friend, but you can be certain that the universe has been indelibly marked by his brief presence, and will bear those marks forever. In this way, he not only lives on; he's immortal. Someday we will be too. If fact, we already are.
I'm very sorry for your loss, Celia. Despite all I've said, it is a loss. You'll never again have his company the way you're used to having it. As you mourn him, however, you may rejoice in what remains of him, which extends forever beyond his coffin. Your friend was who he was, and did what he did, and nobody can ever take that away.
It is sad to think that many people who live selfishly will never be punished for it in their lifetimes, and then receive no punishment after death. That's why it's important to work to bring them to account while they're alive. The idea that it would be right, or just, for them to be judged when they die will not make it happen. It can be nicer to imagine, but that doesn't make it true. It's up to living people to uphold justice as we see it. Take it as motivation, not discouragement.
Generally our personal beliefs and values are passed on to us by our parents and friends, and reinforced by scoiety. Fair or not, that's how it is. You always have the option to go against those beliefs and values, but then you're going against your family, your friends, your society and often your own instincts. It is possible, and on rare occasions it's the best thing to do. It's what most religious people face when they want to leave their religions, for example.
There's no afterlife in an atheist's worldview. That's why I value life so much. Since there's no reward waiting for me, I work to have my rewards while I'm still here. That needn't mean living selfishly, as charity and altruism can be their own reward. If I believed in a heavenly afterlife I could reach, I might be satisfied with a less wonderful life because I'd think something better was coming. Again, the fact that it's a nice idea does not make it true. So I try to make my heaven right here.
Short answer: If there's no Rapture, we will all die in the end, and permanently. Saying so will not make the Rapture happen. We deal with it.
We might escape the solar system before the sun dies, and even if Earth-based life dies out it could start again somewhere else, but eventually the whole universe will be too hot or too cold to support life and anything remaining will die. There might be other universes (concurrent or subsequent) where new life emerges, but let's not get into that.
We live in a changing universe where life-friendly times and places are fleeting. Our chances of survival reduce towards zero the farther you look into the future. Sorry.
You have it right: since eternal life is the invention of religions, after death we will not exist, just like before we were born.
Does this frighten us? Sometimes, because it's difficult to even conceive of not existing. That's my best guess at why religions claim to offer eternal life: the alternative may be literally unthinkable, even if true.
Nevertheless we accept our mortality, and it drives us to make the most of the single, finite life we each have.
Have a nice day. I'm sure trying to.