Like orangekid stated, it should be no problem getting the 3G working on T-Mobile. You'll just need to jailbreak and unlock it. Please be aware that the iPhone radio frequencies do not include the 1700 MHz band needed for third generation T-Mobile data speeds. In turn, she'll only be able to get 150-200 kbps download speeds max (2G speed).
Good things to know for the jailbreak, unlock, and T-Mobile switch include the iOS, bootloader, and baseband versions, as well as the IMEI numbers. You can find all but the bootloader version in the iPhone's Settings > General > About section. The baseband will be called Modem Firmware there. For the bootloader version, I use f0recast by iH8sn0w at
http://ih8sn0w.com/index.php/welcome.snow .
Best to all,
R