Install all available microsoft service packs and rollups for office and the operating system. If the problem still occurs and she is using cached mode, close outlook, delete the .ost and re-open outlook. This will rebuild the .ost file. If this does not resolve the issue uninstall/re-install office and re-patch. If no good after that ask if the exchange servers are patched to the lastest service packs and rollups and ask for their assistance. If the Exchange serves are patched they could try moving the mailbox to see if this resolves the problem, but only after you have done all you can on the application side.