Hey Bernadette P,
You can view and print the audit history of every transaction in QuickBooks which shows the date and timestamp of each edit to a transaction, including each time it was emailed to a customer.
To see this, open the invoice and go to More > Audit history > Show all. When the email has been sent this will show as Sent: Sent Last Sent: (Date/timestamp).
To make sure your customer receives future emails from QuickBooks, you can have them try the steps on our guide here :)
Thanks!