(If this is not too late...)
Message Tracking should be enabled by default. So unless you disabled it (intentionally) for some reason, you should be able to see if the message was DELIVERED. SEEN is another matter (and DELETED).
Open EMC (Management Console)
Toolbox
Mail Flow Tools
Message Tracking
Enter the Recipient email address
Select the "Receive" option
Doing this, I can see messages that where delivered to me but that I deleted later (that are no longer in my inbox).
If you can logon as the user, you could look for the message in deleted items or in "Recover Deleted items" under "Tools" in Outlook (assuming Outlook is being used - there's something comparable in OWA).
You might be able to see all this from the server end, but I'm not sure how. I don't believe a cmdlet like Get-mailBoxFolderStatistics would show anything but the number of messages in various folders (and not the messages themselves).