Mac Mini locked through Find My iPhone, can't unlock it
When your computer gets locked this way, you need to hold down the option key while booting in order to get access to your computer.  You will select the drive it is on and then enter a password to unlock it.

In the case where that doesn't allow you to fix the issue, you may have an issue with your hard drive. When the folder picture appears in the way it does it can mean that the computer can't find the OS to boot.

This can be because of a few things:

It can't find the drive with the OS to boot


OS got corrupted
Part of the drive where the OS is stored has failed
The Hard Drive went bad or is failing
Worst case: The logic board is failing


You should have him hold down the option key while booting and see if it finds his hard drive, if it does, have him select it and see if it boots.  If that fixes it, great.

I would still have him see a Genius so that they can check his drive to see if it is failing.  If he is under warranty and it is failing, they pop a new drive in for him for free as long as the computer isn't physically damaged.

This has happened to me, the option key thing while booting got me around it until I could get my drive replaced at the Apple Store.

I hope this answer helps you out.