Whether high sea sales needs to be reported under GSTR-1? If so under which headings it has to report?

Reply—No need to report following details in GST returns becuase these are not supply.The following transactions to be treated as no supply (no tax payable) under Schedule III:

a. Supply of goods from a place in the non-taxable territory to another place in the non-taxable territory without such goods entering into India. 

b. Supply of warehoused goods to any person before clearance for home consumption; and

c. Supply of goods by the consignee to any other person, by endorsement of documents of title to the goods, after the goods have been dispatched from the port of origin located outside India but before clearance for home consumption.(High sea sale)

Therefore your client is not required to show amount of High sea sale while filing GSTR-1.