It would be nice if developers could simply override a specific file within a plugin from within their theme, but I'm pretty sure WP doesn't work that way (At least not for overriding plugins. Theme files? Yes. Plugins? No).
If the plugin developer was nice they will have given you some override capabilities like using action hooks, filters, or including their function as static within a class.
From the looks of the events-manager plugin file there are three such action hooks available:
do_action('em_events_admin_bookings_header', $EM_Event);
do_action('em_event_edit_ticket_td', $EM_Ticket);
do_action('em_events_admin_bookings_footer', $EM_Event);
You will either need to hook into these actions to make your adjustments (highly recommended), or duplicate the plugin, rename it, and edit it manually (which means you will need to duplicate these edits every time you upgrade... YUCK!)
EDIT after further researching the events-manager plugin:
While WP doesn't provide this template override functionality, it looks like the plugin does. However after some digging in the documentation I noticed that this functionality doesn't specify weather it supports child themes. Try placing the template override within twentytwelve instead of twentytwelve-child. If that works, then maybe you could move that folder back into twentytwelve-child and create a symlink within twentytwelve to the real folder in twentytwelve-child (sort of tricking the plugin). Doing it this way means you have to recreate the symlink each time you update twentytwelve, but the trade off is that you can now override templates and won't loose your changes if you update twentytwelve (just the symlink).
