I'm sorry to hear you're having trouble with this! Ultimately the whitelisting entry should suffice, but I wonder if the protocol involved might be the culprit. Is this user synchronizing via MAPI/EAS/EWS? The reason I ask is that these protocols utilize the web interface ports and so may not be covered under the whitelisting you've added for them. I'm going to test this scenario internally as well and will let you know what I come up with guidance-wise.
Kyle Kerst
IT Coordinator
SmarterTools Inc.
www.smartertools.com