Presumably he did have root access, but gaining it requires a working sudoers file (as does installing packages, etc) so once you've messed it up then you're stuffed.

R&D mode is a possibility, otherwise a reflash will be required.

Incidentally, you shouldn't edit /etc/sudoers directly - you should create a new file (or edit an existing one) in /etc/sudoers.d and the run update-sudoers. That includes a check for correctness, so should prevent this from happening. If you do need to edit the file directly, use visudo which will, again, check the file before saving it.