You can try erasing it to remove the passcode but you may also find the employee has their AppleID in the Find My feature and that will require looking at the second link below.
Read the Apple Support document about what to do if you forgot the passcode for your iPhone, iPad, or iPod touch, or your device is disabled. Click here --> http://support.apple.com/HT204306
Mar. 2016 post by Niel - https://discussions.apple.com/message/29990012#29990012 - "You can either take it and its purchase receipt to a physical Apple Store, or if you associated it with Apple Configurator, bypass the lock from there."
Future considerations: Use MDM to manage Activation Lock and Lost Mode - "Learn how to manage Find My iPhone Activation Lock and Lost Mode on supervised devices with Mobile Device Management (MDM). This article is intended for system administrators at businesses and educational institutions." - https://support.apple.com/HT202804