To add or remove Focus by TeenDrive from your driver's iPhone, their iPhone must "Trust" your computer.
Follow the steps below to have the driver's phone "Trust" your computer:
On the driver's phone
Plug he driver's phone into your computer, unlock the phone and tap "Trust" on this pop-up when it appears:
Note: if you don't see the pop-up unplug the iPhone from the computer. Then on the iPhone go to Settings > General > Reset then tap on "Reset Location & Privacy". Enter the iPhone passcode and confirm the reset. Then plug it in to the computer and check for the pop-up.
Then enter the phone's passcode on this screen:
On your computer
Open iTunes on your computer and look for this pop-up. Click "Continue".
Your driver's phone should now "Trust" your computer.