Hey Robson, In this case it depends. If it was an independent contractor, then perhaps "collaborate" would be fine. But if we are speaking about a fellow employee of a company, we could say "he is no longer working with us," or "he is no longer a colleague of ours." (the second option carries a more negative undertone, whereas the first is neutral).

I would not use that word. "Collaborator" is a special word that should be used with care. It has many possible connotations you wouldn't want. It is also just too fancy a word for saying something so simple. Say something like: "He no longer works with us" "He has left his job here" "He's not here any longer" "He doesn't work here any more" "He has gone on to another job."