I am planning on sending my ultrasn0w iPhone 3G to my country for my cousin, and the 3G frequency the carrier is using is WCDMA 2100. Will the 3G data work or will he have to turn it off?
Yes. iPhone will not work with WCDMA or any sort of CDMA although it's not ruled out that future devices may support them the current 3 iPhones will not and cannot work with (W)CDMA signals. No future software update can or will support this, as new hardware is 100% required.