Prompted to enter system lock PIN code to unlock Mac
You have only two options:


Convince your former terminated employee to provide the 6 digit PIN. If you have a legal department or legal consult, they may be able to provide additional convincing with threat of a civil suit.
Schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase. (apple.com - How to set a firmware password on your Mac) If you choose this option, then you better hurry because Apple says "You can look up your passcode for a limited time".


Although first-party Apple Stores are closed right now because of the 2019 coronavirus pandemic, some Authorized Apple Service Providers are still open. You can find one here: https://getsupport.apple.com/



This happened because your former employee attached an iCloud account to the Mac, enabled the Find my Mac feature, and then initiated a remote lock sequence from iCloud. The lock status is stored in the system firmware, and cannot be bypassed even by replacing the boot disk.

Apple has a great page showing all the possible boot screens for a modern Mac.