/**
 * Get the author post count for a tax query.
 *
 * @link    http://wordpress.stackexchange.com/q/159160/1685
 *
 * @param   array   $tax_query
 * @return  int
 */
function wpse_159160_get_author_posts_by_tax( $tax_query ) {
    global $wpdb;

    $where = get_posts_by_author_sql( \'post\', true, get_post()->post_author );

    $tax_query = new WP_Tax_Query( $tax_query );
    $sql = $tax_query->get_sql( $wpdb->posts, \'ID\' );

    return ( int ) $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->posts {$sql[\'join\']} $where {$sql[\'where\']}" );
}

$count = wpse_159160_get_author_posts_by_tax(
    array(
        array(
            \'taxonomy\' => \'highlight\',
            \'terms\'    => \'featured\',
            \'field\'    => \'slug\',
        )
    )
);

echo "$count Featured Posts";