If he has a TMobile sim card, that would be normally locked to
TMobile, with automatic roaming on to Cingular where TMobile is not
available, is how I understand GSM to work.  From what I have seen
here and in other ngs, you cannot change from TMobile if TMobile is
present.

Of course this all depends a) on the SIM card, and b) any roaming
agreements.

For instance I have a Tel$tra Australia SIM, in LA & Orange counties
in California it roams on Cingular and the now defunct AT&T network.
Although TMobile is present (sharing Cingular network maybe?) I cannot
roam on TMobile.  By that I mean TMobile rejects my log on request.

However in other parts of USA TMobile shows.  Go figure!

Of course, YMMV!