Here is a test you can try:
1. Send the user an email
2. See if Pending mail increases for the user
3. Look in the MAGT log for that user name around the the time you sent the message
You might see a line referencing Still handled by Desktop. He might have inadvertantly installed Dekstop Manager with the Redirector option and now he can't receive from the BES because it thinks the Redirector should be doing it.
I would try that test and see what the logs show.