A customer says their email never came, but the log says Sent

Open the message from the log by clicking its subject. It shows exactly who it went to, which plugin sent it, and what the mail server said. If the address is wrong, or they want it again, use Send again on that page: change the address if you need to and send. It goes out as a new message and the log links the two. If the first one was still waiting to be tried again at the wrong address, it stops being tried.

That needs the message itself to have been kept, which is off unless you switch on Keep the message itself as well under Settings → What is kept. With it off you still see everything else about the message, but there is nothing to send again. A password reset or new-account email, from WordPress or WooCommerce, can only be sent back to the address it first went to, and the page shows it with the key in its link hidden, so nobody can copy the link from the log.

"Sent" means the mail service accepted the message. If it is not in the customer's inbox, ask them to look in their spam folder.