He may be using a iphone 3g or 3gs on t-mobile but he isn't using t-mobiles 3G network. The ATT iPhone is configured to use 1800MHz for their 3G network. TMobile uses 1900MHz, there is no possible way to change this.
I'm using Simyo in Germany, prepaid + €9.90 per month unlimited data speed up to 1 GB then throttles down to Edge. I've never used more than a GB in a month.