Has anyone figured a way for this plugin to be responsive?

Are you talking about the event popup that does not get resized when screen is smaller ?

If this is the case, you can add this modification at the end the plugin file css/custom.css

@media (max-width: 568px) {
   .simplemodal-container {
       height: auto !important;
       width: 100% !important;
       position: absolute !important;
   }
}

A model is here https://github.com/franceimage/ajax-event-calendar/blob/master/css/custom.css

I hope this helps

FranceImage, that was perfect and worked like a charm.

Thank you sooooo much!!!