Document is checked out by a user no longer with the business - how do we undo the check out?

An admin should be able to right-click on the object and select "Undo checkout".  You'll lose that version (so any uncommitted changes), but it should then be rolled back to the latest checked-in version.

Maybe I am wrong, but a missing thing is the force check-in of the document. Sometimes we had cases where users make a lot of changes on certain documents or metadata card and the same thing happened like LeaJoan case but there was important info saved in the document. Unfortunately, the data was lost because the document was unable to force check in from the admin side.

Well you can’t force checkin a document, as the document is cached on the comouter where the editing is happening, sonthe checkin must be done from the user side. 

The only way to unlock it from admin side is to force undo checkout which will revert it the last version saved in M-Files.

Craig Hawker Thankyou - our Admin user has now completed this :) 

I'm aware of that. I just had an idea for a feature request since we have contract templates and sometimes users get lost in the entire M-Files and we have to check it in for them, it would be much easier from Admin side to check in the document and overwrite the local users. Maybe a possibility to have "admin approval" for a check in by the original user and add an additional built in property which contains info about who forced the check in, so everything is transparent.

Edit:
and also to add to the feature I mentioned that the document would be saved on the users side and do an automatic check in after that.

You can enable automatic checkin on document close, on client side