You can't leave him alone and hope it works out. He has mental issues that need managing, and after dark those issues aren't being managed in a helpful way. You

You can't leave him alone and hope it works out. He has mental issues that need managing, and after dark those issues aren't being managed in a helpful way. You could just as well say you don't need design or a coding standard or SCM or CI and just hope it works out. Same principle. He's reluctant to move out because the office has become his haven of sanity and security, where the other people help ground him and distract him from his issues that creep back like demons in the night. Most likely he's getting more and more scared as this goes on, helplessly watching this slow motion train wreck progress, fearing the worst and also feeling powerless to stop it. If you just leave things as-is, he'll slowly degenerate and will eventually start having problems during the daytime as well, which will make his co-workers fear and dislike him, and that will spell the end. He needs help from a trained professional. Helping him get that professional help, and then being a support while he's getting that help is the most promising approach to keep him on the team. He WILL need to move out at some point (soon), and he WILL need a support group and also people he can just be with (loneliness exacerbates the problem). Get two appointments with a professional after explaining your situation. Send him in alone to the first one, take him home, then go back for the second session to talk to the professional about their opinion on how and if you can make this work. Ideally you should do this twice in order to reduce the chance of a professional who's bad at their job. You have to decide how far you're willing to go (after talking to a professional) and make those boundaries clear, because otherwise you're needlessly taking your entire company down choppy waters that will harm him, you, and your company.