You can try running outlook.exe /resetfolders from the DOS prompt on that users PC. If not you can try to use MFCMAPI to correct the folder id.
This issue is caised by the Notes folder id on Exchange somehow changing and the BES no longer recognizes it, as the BES is only concerned with the default folders in Exchange.
You can also search for ConstructPIMFolder here,
BlackBerry Technical Solution Center, there are artciles about using MFCMAPI to see if the folder ids are different. If you, you should be able to correct it by copying pasting the correct folder id.
But be warned, MFCMAPI when used wrong can royally screw up a mailbox.