Some context is needed for why I am asking this question.
Last month, someone I worked with over a number of years decided he did not want to continue in this world and ended his life.
I probably had not spoken to him for 6 months. We meet or spoke when work needs brought us together. There was always a warmth in our greeting and a laugh. Over the years he got some of the really hard accounts to manage and he had the vision to ask for the type of help I can offer. In 1998 he dealt with the only complaint by a customer against me where I offered to visit them on a saturday morning (at that time I worked mon-fri in Camberly in the Solution Centre, so it was not a matter of popping in) to have a look at a long running issue. He dealt with this curved ball with a mix of professionalism and "I have this idiot who has complained you offered to help them above and beyond and the clown has complained about it".
I have no insight into why he decided he did not want to go on and when I read an email about him I 1st thought "its a common name, there most be an other ...... ......". Sadly not.
So my answer to when a work colleague becomes is friend is when you look forward to your interaction with them and their company when the job brings you together. Sun Microsystems was a very rich place according to this definition.