Line around 110

new-class-addify-edit-order-by-customer-front.php

<?php

foreach ($billing_fields as $field_key => $field) {
                    if (! in_array($field_key, (array) $billing_all_fields, true)) {
                        continue;
                    }

                    $function_name = 'get_' . $field_key;

                    if ($order->get_meta('_' . $field_key)) {
                        $data = $order->get_meta('_' . $field_key);
                    } else {
                        $data = $order->$function_name();
                    }

                    woocommerce_form_field($field_key, $field, $data);
}

old-class-addify-edit-order-by-customer-front.php

<?php

foreach ( $billing_fields as $field_key => $field ) {

					if ( ! in_array( $field_key, (array) $billing_all_fields, true ) ) {
						continue;
					}

					$function_name = 'get_' . $field_key;
					$data          = $order->$function_name();
					woocommerce_form_field( $field_key, $field, $data );
}

set.php

<?php

foreach ($billing_fields_form as $billing_field_key => $billing_field) {
                if (isset($_POST[ $billing_field_key ])) {
                    if ($order->get_meta('_' . $billing_field_key)) {
                        update_post_meta($order->ID, '_'. $billing_field_key, wp_unslash($_POST[ $billing_field_key ]));
                    } else {
                        call_user_func_array(array( $order, 'set_' . $billing_field_key ), array( sanitize_text_field(wp_unslash($_POST[ $billing_field_key ])) ));
                    }
                }
            }

set_ is missing ;)