I'm inclined to agree that the developer should not have been let go for a mistake - it was the fact that the mistake was allowed to happen and could not be mitigated that is the core issue.

If anything, the existing developers should be reprimanded and the manager fired for letting the situation occur in the first place.

Do a post-mortem, find the root issues (pun intended), and don't let it happen again. Also, take a long hard look at your permissions and security practices!