Acetylene

Genuinely suicidal people commit suicide because they believe that nothing can ever get better: that life is fundamentally unbearable for whatever reason, that it CANNOT be improved, and that no could help them because there is nothing to be done. They maube think it's everyone else's fault for creating an unbearable world, or they may think it's their own fault for being too broken to fit in it.

Hence, people who seem fine get found dead in the basement, and their family and friends wonder why they never asked for help. These people probably didn't want to bother anyone with their "unsolvable" problems, either because their family and friends were part of the problem, or because they were too ashamed of their inability to cope with situations that "normal" people seem to enjoy.

Don't get mad at him or you will make him feel worse. Don't act fake happy or you will shame him. Be professional, but also be kind: give him extra time to complete assignments, and create failsafes to prevent mistakes from slipping through. This is both because he will probably make a lot or errors while recovering and because if others find out about his mistakes and scold or discipline him, it may worsen his feelings of despair and worthlessness.

Basically, set him up to succeed at work, and give him reasons to feel competent, useful, and worthwhile. This is the best thing you can do for him, as a co-worker. People who feel useful are much less likely to commit suicide (again).

Edit:
Forgot this part.

DON'T ask him why he did it, or try to "understand". You will NOT understand, unless you are depressed yourself and have considered suicide at some point and rememebr what it felt like - and if that's the case, then you don't need to ask because you already know.

If you do ask, and he does answer (which is unlikely), you will be shocked at how minor his reasons seem to you. You will say something like, "Why didn't you just do x?" or "What's so hard about x?" or "I could have fixed that, why didn't you just ask?" and make it a hundred times worse for showing him, once again, how pathetic he is.

Sample answers (from my own experiences or that of my friends):
"Nothing I do matters. My life is meaningless and worthless so I might as well die."
"I am unlovable and no one will ever want me. I will be alone forever and I can't bear that."
"I mess up everything. People would be better off without me."
"Nothing is wrong. Everything is fine. I don't know why I hurt so much but I do and I couldn't bear it another minute."

You aren't going to learn anything that you want to know, so please don't make him re-live it to satisfy your own curiosity.

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There's no justice. There's just us.