did "don't stop' say he was an existing customer?
if he has Tmobile then you just move the sim card from one device to the other, and thats it.
if the sim card isn't the same size then you would have to get them to change the sim card number on the account
if he isn't a t-mobile customer and he is porting from another carrier, you would just need to bring your iPhone , t-mobile sim card (if you don't have one they will charge $15) , and give them your old telephone number, account number, and any billing passwords from the other carrier and they will port your service over and cancel your old carriers service
the t-mo website or the store should be able to order whatever size you want and send it to you and finance it if you want
when buying a t-mobile phone from a t-mobiel store, it will be locked to t-mobile , even if you pay full price, they will unlock your phone if its paid off, last year t-mobile had this weird deal back in September where they wanted the phone on the network for 40 days before they would agree to unlock it
when buying a t-mobile phone from the apple store, it will be unlocked
nobody knows what apple is doing.
is apple going to release a new iPhone that is "AT&T / verizon / t-mobile " and also later sell a sim free unit that works on those 3 carriers plus sprint like it is now with the sim free unit or is going to be like last year when they waited till january to sell the "sim free" version that you can bring to sprint ??
most people who try and second guess apple for a living think that it will be an apple sim
and it will work on sprint
the AT&T verizon and t-mobile iPhone initially wasn't officially sanctioned by verizon, but it is now