If you have the original receipt or the device is enrolled in Apple Business/School Manager, Apple can assist. If neither applies, you will need to ask the former employee for assistance. If the former employee is unwilling or unable to assist, you have a paperweight.
Most Mobile Device Management (MDM) platforms have the ability to remove Activation Lock either remotely or with an escrowed bypass code for enrolled devices. Apple Business/School Manager will be able to remove Activation Lock in the future as well (per an announcement from Apple during WWDC). If you do not have ABM/ASM and/or MDM, I encourage you to begin that process.