Recover root access after wrong usermod -g in Ubuntu 14

I have to recover root access to an Ubuntu 14 box where there is only a single local user and usermod -g somegroup ... has been executed instead of usermod -G ... so the user can't sudo anymore.

I am aware I can boot with a live distro, mount local volume and chroot into the system to change /etc/group but I was wondering if there is a smoothest solution. Ubuntu is running on bare metal. I have physical access to the server and it can be shutdown in a scheduled maintenance window.

Regarding the solution I am aware of, I'm not sure how to perform it because the system has / volume on an mdadm raid 1 array and the live distro would see disks as standalone sata devices.

Any clarification or any other possible solution will be highly appreciated.