Will an iPhone from the UK work in the USA

It will if you specifically buy it from an Apple store and you specifically ask for an unlocked phone. DO NOT buy it from Carphone Warehouse! Their phones are locked (but they lie about it).

Note, however, that the warranty will only be valid in the UK, so it will have to be returned to the UK if it requires service.

The other useful bit of information is that the price will be very substantially lower in the US because there is no VAT. So you might consider buying it when you arrive in the US from an Apple store. That also solves the warranty problem.

Phones purchased in Europe are designed to work on GSM networks.

America has GSM networks, but we also have carriers that use CDMA only - and the GSM phone will not work on CDMA.

Find out who your friend's wireless carrier is, and determine if it's a GSM or CDMA carrier. If it's a CDMA carrier, I'd wait until you get to the US to purchase the phone.

It is well established that they sell phones that lock to the first UK carrier the are activated with, that they cannot be used with a non-UK carrier, and that the sales people will lie and say they are unlocked phones.
[Edited by Host]

Lawrence Finch wrote:

"...DO NOT buy it from Carphone Warehouse! Their phones are locked (but they lie about it)..."

Probably not necessary to make disparaging remarks about retailers. Certainly not necessary to make unsubstantiated accusations about a company.