Since the title says "Ex-Admin", revoking credentials when someone leaves the company is a must. If there's centralized auth (AD), this is pretty straightforward. If not, then at the very least it makes sense to use Puppet, Ansible, etc. to lock that person's account on all production servers.

Beyond that, be sure to keep regular backups (and test them), and audit all user actions. (feed the logs into something like Splunk, running on a separate machine)