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.