<?php
add_filter( 'manage_edit-shop_order_columns', 'woo_customer_order_coupon_column_for_orders' );
function woo_customer_order_coupon_column_for_orders( $columns ) {
    $new_columns = array();

    foreach ( $columns as $column_key => $column_label ) {
        if ( 'order_total' === $column_key ) {
            $new_columns['order_coupons'] = __('Coupons', 'woocommerce');
        }

        $new_columns[$column_key] = $column_label;
    }
    return $new_columns;
}

add_action( 'manage_shop_order_posts_custom_column' , 'woo_display_customer_order_coupon_in_column_for_orders' );
function woo_display_customer_order_coupon_in_column_for_orders( $column ) {
    global $the_order, $post;
    if( $column  == 'order_coupons' ) {
        if( $coupons = $the_order->get_coupon_codes() ) {
            echo implode(', ', $coupons) . ' ('.count($coupons).')';
        } else {
            echo '<small><em>'. __('No coupons') . '</em></small>';
        }
    }
}