sorry – little bit late to the party, and this is not the proper way of doing this, but if the plugin is no longer being developed… this does the job:
fine the file
wp-content/plugins/woocommerce-delivery-notes/includes/class-wcdn-theme.php
and change line 102 from:
if( $order->billing_email && !$sent_to_admin ) {
to:
if( $order->billing_email ) {
then you should be good
