Maybe try a spare Outlook 2010 machine and see if that synchs this mailbox successfully or not to OST.
Was his machine upgraded from a previous version of office or not? maybe you need to check the MaxLargeFileSize reg keys and ensure the users machine is set for 50Gb OST limits:
http://support.microsoft.com/kb/832925I don't see any reason why you can't buy a single E21k0 Ent CAL...