The client would still see the invoice and attachments on their email, even if the transaction was already deleted in QuickBooks Online, David.
Please note that deleting a transaction does not recall or reverse the sent email. However, they'll get a message that says "Invoice deleted" once they click Print or Save.
Feel free to check out this article in case you want to restore an accidentally deleted transaction: Use the audit log to re-enter deleted transactions in QuickBooks Online.
Please know that the entire forum is always ready to help if you have more questions for us.
