He needs something (a quad band phone, in fact) with a SIM-card. That's AT&T or T-mobile, if he wants to buy it in America.

There are so many plans (pay as you go) that's it's impossible to tell which one is best. That also depends on how much phone calls, how much SMS and how much Mb data traffic he expects.
However, since most contracts last one year, it's better to have an American contract and use that in Europe for a few months than the other way around.