The best way to get a friend off a phone or to just annoy one, is to shut down their phone while there not looking ^_^
The best way to do this is to talk with ur friends to set up distractions (make them put there phone down and look away) if you can pull this off sucessfully without getting caught at minimum of-
4 times it should start to bug them. Eventually (depending on how strong willed they are) it should deter them from using their device as much or at all. Some will most likely buy a new phone tho -.-
If you know your friends password the easiest way to stop them from being addicted to their phone is to change the password. But you already know that one.
The next step is to take a screen shot of the home page of your friends device and make it the background of the homepage then move all the apps to a separate page.
Change the sound limit to nothing
Change the sound to mono and the bar to left, so that even after you have changed the sound limit your friend still has problems.
If these steps result in loss of friendship or fights ect I will take no credit, it is your choice to perform these acts not mine.