I'm not sure how they could miss the debug code written on the top of that page after sending.
The debug code is the actual SMTP log from your server communication, so they need to see that to disprove their assumptions.
Tell them to make sure they are entering a TO, Subject, and BODY as well as selecting the FROM source. If they aren't all filled in then it won't send. That is probably why they don't see the debug or get the email.