Company were also stupid
They sack a sysadmin without protecting their system.
Sack someone and you make an enemy. As soon as you are considering it make sure that you can protect your system.
Bring them into the meeting, chat about why and how sorry you are to let them go. Offer a nice (but honest) reference. Keep them there.
While this is happening change all passwords, temporarly block all remote access, make sure your backups are OK.
If there is no new sysadmin ready to drop in, oh dear!
Remember there are people who think BOFH is a documentry.