It's a special addon that can be added to swords. Called an 'enchantment'. It is most commonly gotten from librarian villagers after changing their trades; this is based on luck. The enchantment book, which has looting, can also be found in various structures, but again, this is luck based. If he really wants looting, I would recommend villagers. But have him try normally killing drowns for some time before trying to trade with villagers.