Verizon is on a type of technology called CDMA which is used by Sprint, Nextel, Alltel, and Cricket among others. CDMA is completely incompatible with the other major digital wireless technology used in the US (and most of the rest of the world) called GSM. In the US, GSM is used by AT&T and T-Mobile and a few smaller carriers.
You can't use a GSM phone - like an iPhone - on a CDMA carrier. They use fundamentally different signalling technologies and are completely incompatible in terms of frequencies, protocols, and signalling methods. This is not hackable, and it can't be gotten around. Trying to get a GSM phone to work on a CDMA network is a lot like trying to play an HD-DVD disk on a Blu-ray player. It's pretty much impossible.
You friend can hack an iPhone and use it as an expensive iPod Touch, but it's not going to work on Verizon.
Bricking refers to earlier problems with hacking an iPhone to work on T-Mobile (or Rogers in Canada) and having firmware updating issues that resulted in a dead phone. All bricked iPhones can be "de-bricked" by upgrading them to firmware release 1.1.3.
As Random mentioned, you can hack iPhones to work on T-Mobile in the US. This is really easy nowadays. In fact, there's a YouTube video showing a guy walking in with an iPhone, unboxing it, downloading the software off the internet and unlocking it and getting it working on T-Mobile al in less than 3 minutes. And he doesn't even look particularly rushed while he does it. But T-Mobile is on GSM... so this is really just opening up the software on the phone to allow it to read the SIM card from T-Mobile and access T-Mobile's network.