Can't log out of someone else's find my iphone without their icloud password
Contact the pervious owner and ask them to log into iCloud themselves and release this phone. 


https://support.apple.com/kb/PH2702


Failing that, ask if work has this device enrolled in an MDM or to contact Apple to validate via the serial number that the device is not owned by a consumer or different company. In that case, your IT department can bypass this activation lock. If work has an HR department, they can request / require the previous owner to follow Apple's steps to release the device. Legal could bill the past employee for the replacement cost. 


How secure is Activation Lock on iOS 7 (and later)?


You probably need to just turn the device in or wait it out since bypassing the lock isn't viable.