It's exactly wrong for without showing Sales bills in GSTR 1....! But, No any other way except above said reply... Show the details in next* GSTR 1. In case in final GST Audit AO can impose any penalties...
Do not show in next month GSTR 1. Tell the client to add in his GSTR 2A. This will get reflected in your GSTR 1A. Accept this invoice added by client and the mistake will be rectified.