The software is just the messenger that sends the banking info to IRS, it was no clue whether the payment has actually been made or not,  your client should be checking that the payment was actually withdrawn from the account, just like if they mailed in a check....they'd want to check to see if/when it cleared their account

You can call the EFW phone number and inquire about a payment....Call IRS e-file Payment Services 24/7 at 1-888-353-4537 to inquire about or cancel your payment, but please wait 7 to 10 days after your return was accepted before calling.