1. Take a screenshot of your friend's home screen (press the Power + Home buttons at the same time)
2. Hold the Home button until their apps start wiggling.
3. Drag all their Home Screen apps to the right onto another screen. Press the Home button when done to stop the wiggling.
4. Go to Settings > Wallpapers & Brightness > Choose A New Wallpaper and set their Home Screen wallpaper to the screenshot you just took.
Now, when they tap the apps on their home screen, nothing will happen.