on demand and ppv shows will go away on their own, but not regularly recorded shows. if the dvr is getting full and you have a recording set to delete when "disk is full", it will erase the oldest unprotected recording to make room for a new recording. you can stop shows from automatically being deleted by setting them to the "i delete it" option.
also, when you set up a series to record, you can set how many episodes of that show are saved, including save all of the episodes. if she had it set to only 5 episodes, and new episodes recorded it may have deleted the older ones to make room for the new ones.