Text messaging problem
You should try to isolate the problem. If you can, confirm that you can send messages to other cell phones that are not VZW phones. Ideally, you would want to try to send a message to another T-Mobile phone. You can also send a message to an e-mail address.
If you can do all this stuff, then the problem is on the other end. Your buddy may have inadvertently blocked messages from your phone number. Or something could be wrong on T-Mobile's network, at least with respect to his text messaging service.
Bear