Did she delete and then re-add her email address from her phone, either in an attempt to fix a problem or during an OS reload? As soon as you delete an email address, the calendar gets deleted as well and all of the events get moved to the "Device Default" calendar. When you add it back, the calendar comes back, but it doesn't automatically copy the events back to the old calendar again.
When this happens, you can go into the options menu in the calendar and type "move" (without the quotes), which will prompt you to move the events back to whichever calendar is set as the default (not the same as the device default calendar you are talking about, read below). However, since she is syncing her events back to the correct calendar, you should be able to delete all of the events in the "Device Default" calendar and they shouldn't come back.
One more thing about default calendars: There is also a setting to select the default calendar under options / advanced options / default services. This has absolutely nothing to do with the "Device Default" calendar that you are talking about, but instead is how you decide which calendar is automatically selected when you create a new event. It is also used by some sync software to decide where to put new events.
06-03-10 07:30 PM