you can efile the extension through the software and have his account debited or safer really as far as covering yourself he can make the payment at irs.gov

The same way you would for an irregular client.  Sorry, it’s been a long day.  But as a side note, I had an issue with a couple of returns that had extensions that had balance dues being paid with the extension.  I had to uncheck the boxes for e-filing the 1040 to get the extension to convert for e-filing to get it to transmit —— just in case you run into the issue.