You will need to get the TMo Pearl unlocked for use on another carrier. If you are a TMo customer they can do it for you. If not, then you'll need to contact one of the unlocking services listed in our sponsor section.
Also, I moved your post to a new thread as the one you posted in is supposed to be kept free of these questions.
Yes. If your friend has been a customer in good standing for 90 days they should unlock it for him.
Changing the OS to Cingular is not necessary to use it with Cingular. It will, however, give you the latest OS available for the Pearl, since Cingular has the latest OS version release right now. It won't change the branding of the phone. Once a TMobile BB, always a TMobile BB. But once you get it unlocked it will function on Cingular as if it were a Cingular BB. Does that make sense?