Sudo command no longer functions properly
You should be able to get round this by rebooting into single user mode...


Reboot the computer holding CommandS
The command line should come up showing something about how to mount the drive in write mode mount -uw /, follow these steps.
Now you should be a root user able to edit the /etc/sudoers file.
Issue reboot now to exit and reboot into normal mode.


I don't have a second Mac to test these instructions are step-by-step complete and obviously can't write on the machine while it's rebooting, so please let me know if you get stuck on any steps.