@bez252 Actually what they describe might also work for you, based on a quick glance through the setup code.
To shutdown from the node_helper you need to change some code in the file(will be in MMM-Button/node_helper.js in your file system).
Here is an example from my module to shut down the RPi, you can replace the self.sendSocketNotification(...) in the node_helper with a similar command:
exec('sudo shutdown -h now', null);
Also, at the beginning of the file you need to include the exec function, like here:
const exec = require('child_process').exec;