Is he sending it to the correct email address?? Do you not have a "blackhole" email address?? (where all mail is received if it doesnt have a known recipient on your domain) - if so check in there...
Pete, I had this a few months ago and my server was blocked? I contacted my host and all cleared with the flick of a switch - good luck!
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Could be (a) his ip address or the ip address of his ISP's mail server is blacklisted by your ISP as a spamming address, (b) the mail queue at your ISP had a problem and was cleared, (c) it arrived at your end but was marked as spam by your mail client, or (d) your ISP's mail server was unavailable for a time.
If he sent the email and received nothing back it's most likely a,b or c. If he received a bounce it's either a or d. Try sending an email to yourself from another account (if you have one) and see what happens.