How did he pay?
SELECT t1.payment_method FROM customers AS t1 JOIN customer_orders AS t2 ON t1.customer_id   =   t2.customer_id JOIN order_items AS t3 ON t2.order_id   =   t3.order_id GROUP BY t1.customer_name ORDER BY sum ( t3.order_quantity )  DESC LIMIT 1
payment method refers to t1.payment_method; 'he' implies filtering or grouping by customer (possibly male if gender data is available); how implies the method of payment; grouping by customer_name and ordering by sum of order_quantity to find the top implies looking for the most frequent or highest volume of purchases to deduce the payment method used for the highest quantity of orders.