How do I change Software Update server to official one on OS X Lion (or later)?
It depends on exactly how they changed the update server setting.  If they did the obvious thing, this should delete the setting (so it'll go back to normal):

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL


If that doesn't work, it's possible it's configured as a per-user (rather than system-wide) setting, in which case this should do it:

defaults delete com.apple.SoftwareUpdate CatalogURL