how do you run it on TMobile?
edit: You are not in the U.S. huh?
FYI, since I see this error a lot. You can run
unlocked iPhones on TMobile (even in the US), you are just stuck on EDGE and can't get 3G, because the iPhone doesn't support TMobile's 3G carrier frequencies. To be clear, unlocked is NOT the same thing as Jailbroken. You can BUY unlocked phones from Apple, and various jurisdictions require them to sell them (or offer them) unlocked (including Canada).
You can't run a phone "locked" to AT&T on T-Mobile in the US without unlocking the phone first. This is unsupported, of course, but only messes with the baseband firmware, and still isn't the same thing as a Jailbreak.
Of course, in this case, you were right. He wasn't in the US, and Apple does sell through TMobile in lots of countries.