Gst not paid on credit note by client

While filing gstr9 for financial year 19-20 we came to know that one of our client to whome we had issued certain credit notes did not pay the igst , and instead claimed the entire amount to himself.

NOw we have claimed ITC on this already in the year 19-20 but we are not getting that amount in our GSTR 9 annual return of the year.

Please help us as to what is the way out of this... 

1. can the client still upload the igst of credit note of FY 19-20 ? can he amend his return any way ?

2. This not being our mistake , what is the way out for us ? can we upload our credit note to GSTIN. and prove our side ?

Any help will be appreciated