Be very careful in wording your letter to the city. You can and should only state that the insured was insured with carrier name, policy number, effective date, expiration date. End of letter. Do not say there was a lapse in coverage (client could very well have gotten another policy from somewhere else as far as you know - let the client explain to the city). We have this type of thing happen routinely whether it's a general contractor holding a cert, a municipality or the motor vehicle division. You can oly state facts. I never give information out verbally either - you can be sued for saying something wrong.
If they didn't renew with you, then you are not certain that they were bare, you only know they didn't renew with you. That's all you can attest to, and I would simply say that, in writing, and be done with it.
My point is if the City was not named on the policy you can not give them any information about the policy. If they were listed you would simply mail them a copy of the cancellation letter that they should received on cancellation.