Thanks for the speedy reply!
The user is offsite. I was able to get him logged in just now, but it seems we're still running into issues with Keychain and iCloud.
To get him logged in, I asked him to first click on his portrait icon at the login prompt, then choose the option to log in as another user. Luckily, we had a local account that was previously created on this Macbook and I remembered the username and password. After that, he confirmed the Macbook was connected to his home WiFi, logged out, and then successfully logged back into his profile with his new password. Not sure why that worked, but I assume it was the communication between his Macbook and our MDM and/or Active Directory. After he logged in, he received an error about a keychain password but didn't grab a screenshot. Additionally, iCloud seems to have prompted him for his password and when he entered it in, he received a message stating his iCloud account would be locked for about 3 hours. He's now worried about locking his screen at all for fear of being unable to log back in again which is understandable.
I was reading here that we may be able to resolve the issue by resetting his Keychain, but that there may be risks involved and to not do it unless instructed to do so by Apple Support. I'm still looking into it, but appreciate your help so far. Have any further advice here?