Does T-Mobile's network in the US allow an iPhone 3GS work as well as the iPhone 5?
No He will be stuck on IOS 4 most popular apps have support for ios5 or higher
I have a 3g 16gb too old espically for a newbie. Get the 3GS unlocked version if you want to be cheap/ most bang for bucks. He wil be stuck on EDGE until Tmobile finishes adding 3g Support for unlocked iphones. They have done a few cities google which ones 
16/32gb 3GS is the best in my opinion. Favcetime and stuff is impossible over 3g unless you can find a ip4 off contract for sub 200 dollars then get tyhat i used  4 different iphones on tmobile 2g 3g 3gs ip4 been with them since 2001