I'm sorry for your loss.
Someone in a group project I did lost one of her best friends out of nowhere. Died in her sleep. It was gut wrenching to see the disbelief and grief in her and the family/friends involved. I don't know if realizing that would change people's minds - it often isn't a fully rational decision, I think. I can imagine despair already being off the charts so I don't see how guilt can improve that.
At some point life has to go on. At some other point, even though it seems infinitely far away, you will have to give this event and his life a place in your heart to rest. It is, by all means, a difficult process but doing "normal" things is a good first step. It gives your mind some time to process it all.
Best of luck, man.