If your client is not expecting a refund, there is no hurry to file.  No interest or penalty for late payment or filing will be assessed during this period of auto postponement.  However, underpayment of ES-tax penalty will still apply.

Having said that, if your client has filing obligations for other types of returns not covered by Notice 2020-18, an extension should then be filed no later than the usual due date on April 15 if more time is needed.