For the time being, although the payment of taxes due has been deferred to July 15, 2020, in TurboTax you cannot enter a date later than April 15, 2020 for your payment by direct debit.

In order to take advantage of the extension of time to pay your taxes, you should select “I’ll mail a check” as your payment option when filing. 

I tested efiling for intuit from 1995 until 2003.  Unless they changed their code in the efiling it always was that a filer could efile after 4/15.  In this situation he would have the same direct debit date as the efile date.  As an example if he owed money and didn't file until July 15 his direct debit date would be July 15.  However, he could not efile before April 15 and give a direct debit day of July 15.

So if this is still true your customer would have to wait to file at a the later date he wanted his direct debit to be.

TurboTax is preparing an update this week that will allow you to enter July 15, 2020 as the payment date. 

However, if you do not want to wait for the update you may use this procedure. 

"If you haven’t filed your federal taxes with TurboTax yet

Select Pay by check as your payment method and mail it in before July 15, 2020."  Ref: How do I change my payment date?