Issues in Outward Supply

One of Client has filed GSTR 1 with less bills as compare to actual bills say filed only 300 bills instead of 500 Bills due to mistake in 2017-18.
how we should disclose in GSTR 9. 

As the entries are not shown in GSTR 2A, Customer creates problem and demanded the entry in R1 (His GSTR 2A)
how we can resolve the problem 

Outward Supply and GSTR 1