Under law no negative parity can be sought, so if a few cheques were honoured when technically they couldn't have been, he can't take a plea that this cheque too should have been meted out the same behavior.
It's between the Insurance Co and him and you have nothing to do with this.
Engage an able Lawyer dealing in consumer disputes and file a meticulously drafted reply to his complaint.
Let me know incase you need any further assistance.
Best,
Adv. Vibhanshu Srivastava