In order to do this she would have to have enabled iCloud and Find My iPhone on her phone prior to losing it. If she did, you can go to https://www.icloud.com on your computer, logo into her iCloud account and click on Find My iPhone to try to locate it. Unfortunately, thiefs usually know how to restore a stolen iphone as new, removing the iCloud account and settings from the phone, so this still wouldn't allow you to find it. You might this user tip helpful regarding steps to take when your iPhone is stolen: What To Do If Your iDevice Is Lost or Stolen. Good luck.