The code and regs for late payment penalty have not changed. However, the period from the original due date of April 15 to July 15 is "disregarded in the calculation of any interest, penalty, or addition to tax for failure to pay the Federal income taxes postponed by" Notice 2020-17.
Extensions for filing a return do not offer reprieve for late payment penalty. Nevertheless, your client would not be subject to late payment penalty if at least 90% of the tax was paid by April 15 (or July 15 under Notice 2020-17) and the balance due was paid with the return when filed.
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