The USA is unfortunately not like many foreign countries where is is normal to just buy a prepaid SIM card to stick into a GSM phone. Carriers like Cingular and T-Mobile do offer such an option but you have to go in an official company store and ask for it. I believe the setup fee to buy the card is somewhat expensive though, and the calls are pretty expensive too. I had a friend from Canada visit that had to do that recently. He couldn't believe the high cost to just get the SIM card. Once you buy the card though, you basically just stick it in the phone and power it up. Just make sure the phone is UNLOCKED . Most GSM phones in the USA are locked to a specific carrier and you will not be able to use another carrier's SIM card in the phone so be sure to check with your friend first. Unless he is certain the phone is UNLOCKED, you should assume it is locked to whatever carrier provided the phone to him.
As long as the unlocked phone recognizes one of the following 850 MHz;900 MHz;1800 MHz;1900 MHz you should be fine. (You may have to manually select the network from the handset menu)
A T-Mobile SIM card from a corporate retail store (not an authorized dealer) will run you $20.00. The "activation kit" is $50.00, which is comprised of the $20.00 SIM card and $30.00 in airtime. You'll have to have the prepaid service activated for you by the retail store, or you can call prepaid activations at 1-877-778-2107.
Prepaid airtime refill cards come in the following amounts: $10.00 (30 minutes), $25.00 (130 minutes), $50.00 (400 minutes), $100.00 (1,000 minutes).