Delegate can't save meeting changes on shared calendar

I have an assistant who is delegated as an Editor on her manager's shared
Outlook calendar. She has been getting an error message "changes to the
meeting could not be saved. The meeting has been updated by another person.
Close the meeting, then re-open it to make your changes." She needs to
update the location or change the time but she can't. It just keeps popping
up the save changes box and then the error message comes back up. She gets
this error message on meetings that were created by her and on ones that were
created by her boss and on ones that were created by her manager's former
assistant.
Both the assistant and her manager have Windows XP and Outlook 2007 on their
machines.
Any help would be appreciated.
Thanks