The following method is working only for Android phones. Go to Settings > Security and Privacy > Device Administrators. She's probably using an application to hide all other apps and files. Disable all the apps in the Device Administrators list and you should be able to see all the installed apps. Secondly, use an application from Google Play that allows you to hide apps as well, and secure it with a pin, pattern, etc. Install Snapchat and add it to the hidden list.
