Find My Friends App was deleted when update was installed

The Find My Friends features where merged with Find My iPhoen into the new "Find My" app.

You can find the Find My Friends features in the People tab of the new "Find My" app.

Share locations in Find My on your iPhone and other devices - Apple Support

find my friends was deleted from my iPhone 6s during a recent update

The app name has been changed to Find My and it's on your device.