Originally Posted by MX727

Jailbreaking allows you to add applications and access the phone like a disk drive with programs such as iPhone Browser. My phone is jailbroken, but it isn't unlocked. Still on the AT&T network. Very easy for me to take it back to the original state.

Unlocking allows you to use your phone with another carrier and more often than not cannot be undone. You said your friend had unlocked his phone. Now he has bricked his phone.

iPhone Question: “Unlocking” and “Jailbreaking” the same thing? | HighTechDad Blog

Gadgets and You: Iphone activation vs jailbreaking vs unlocking

Here is a link for him to try and get his phone back from the bricked state:

Step-By-Step Guide to ReVirginize or Restore iPhone - iPhone Hacks