HOW DO I GET A JAILBROKE 3G IPHONE WORKING FOR MY GFS TMOBILE ACCOUNT?

She has a jailbroke old iPhone right away and she is with tmobile. My friend figured out how to jailbreak his brand brand new 3g iPhone. If I buy her a brand brand new 3g and have him jailbreak it can she only take the sim label out of her stream iPhone and put it in the jailbroke 3g?