I hope it's not inappropriate to post this here. I (fortunately) have no experience with mental health issues but I am concerned for an (ex) friend of my DH's and I am wondering if someone can shed some light.
My DH and the friend are both tradesmen and after meeting on a job a couple of years ago they realised they could work well as a team and since then have done pretty much every job together. They both worked really hard and built up a brilliant reputation, they were constantly in demand and had work booked in for most of 2012.
The friend is in loads of debt that he's digging his way out of. He also has a bad homelife - fallen out with all his extended family, has a bad relationship with his wife and son, and has no other friends. He seemed to have no joy in his life and was quite unlucky. My DH described him as 'miserable' and 'grumpy' which most people who worked on jobs with them also (independently) said. My DH was really good to him, did lots of favours, he often came and stayed at our house and my DH even took him on holiday. When I met him it was like there was a big black raincloud over his head.
The friend became a bit of a workaholic 6 months ago, working 7 days a week. This seemed to 'break' him and a couple of months ago he ended up in bed for a week. I don't know the ins and outs of it but it seemed to be some kind of virus although I know he was told that he was nearly in a coma and should slow down 
He didn't slow down and continued to work round the clock. AIUI he never got a break at home either as his wife didn't do very much so he would get home from a hard physical day and then have to start making tea and doing chores.
Last week my DH and the friend had a very petty disagreement over nothing - it seems to have been 'led' by the friend, like he was looking for a fight kind of thing. On Monday the friend walked off the job and left my DH to work on his own. He is refusing to speak to my DH or to any of their customers, although he emailed my DH to say "all the work is yours, cancel any you don't want to do or don't have time for" but it's more complicated than that. My DH has no idea what to tell next year's customers.
We are struggling to explain this really odd behaviour. We know he doesn't have another job to go to (although he has got odd bits here and there but nothing like the massive lucrative contracts they had as a team that would have got him out of debt!). I do not understand why he has effectively overnight destroyed the one thing in his life that seemed to be good (i.e. the business and the chance of paying off his debt, and also the friendship with my DH).
My DH is very angry with him for putting him in this position but I think it would be helpful if we could understand why it has happened. One theory I have is that he's had a sort of breakdown and it's related to the period in bed a couple of months ago. I hate to think of the friend sinking lower and lower and nobody helping him. Any thoughts would be appreciated.