Refer to How to remove Activation Lock - Apple Support There are only two ways to remove a lock:
#1 If you have the necessary information, you or the signed in owner can do this yourself by simply signing in with the owner account and password information and doing as instructed in the above article.
However, that is clearly not available to you now, in which case…
#2 Apple can completely reset a device if a person can provide sufficient evidence of valid ownership. Read the requirements and steps for an Activation Lock support request at: https://al-support.apple.com/#/additional-support and links at the bottom right for the type of service requested (you may want to use the business one, but I am not sure). Apple says they “may” require "proof of purchase documentation” and that you need “proof of ownership documentation”, but does not detail what currently constitutes proof other than it “must include the product serial number, IMEI or MEID.” User reports on this community indicate that typically Apple will absolutely require, and ONLY accept, an original receipt issued by Apple or an Apple Authorized Reseller. Presumably this is the only kind Apple can authenticate against fraud since Apple is not involved in third party sales. Apple recommends that you "Keep your receipt after you buy an Apple device" --> Keep your receipt after you buy an Apple device - Apple Support
Note that the device cannot have Lost Mode set on it. If the employee was vindictive they may have done that and there is nothing you can do about it.
For future reference, you may wish to configure your equipment for business use, e.g., Apple Configurator 2 for Mac User Guide - Apple Support
