The best way to give a user access to a single folder is to give all users 'No access' in document settings.
Note: Remember to use the option 'folder and all subfolder and files' so you overwrite the access of all the subfolders too.
You can then add the user to a team and give the team (or the user but team is easier to manage) the access you want to give them to the folder.
Note: The user will need access to any parent folders as well to be able to navigate to their folder.