He'd buy a phone off of Ebay, and it wont have the SIM. My brother has had this happen twice to him. They most likely wont ever be able to use LTE on that device unless they somehow convince the people at a sprint store to trade the device in for one with a UICC SIM, or on more rarer cases: have them contact someone who can order a replacement and be able to 'link' it to the device. This can also be demonstrated by people who have bought sprint phones off of ebay and people don't include the UICC SIM card that came with the device. It is basically just a 'security chip' to allow Sprint and other UICC SIM card using carriers to make sure only authentic devices are allowed to use their LTE networks.

Your phone number is not saved on it, text messages aren't saved on it, contacts aren't saved on it, email addresses are not saved on it. Not only will it simply just not work unless they are using the physical linked device, but there is no information to be retrieved from it what so ever. There is no personal information stored on the UICC SIM card, so there is nothing to be gained by someone who somehow gains physical access to the SIM card.

So, to answer Alex's question: you do not need to lock it, for it is already locked permanently to your device. Though, I'm pretty sure you have to get a UICC SIM from them, which unless things have changed within the last few years, I hear is kind of difficult to get from GSM carriers. So yes, you CAN put an AT&T SIM card into the phone, and have it work, but ONLY if your DEVICE is unlocked by the carrier and thus be allowed to access other networks. You can however, unlock your DEVICE to work with other SIM cards. They are locked to your device PERMANENTLY. You CAN NOT take the SIM card, and put it into another device, even if its the same exact model, and expect it to work. They have a 'personalized code' that is linked to your device, and will ONLY work on that device. UICC SIM cards, are simply there to give you access to LTE, nothing more, nothing less. But, what they don't seem to understand, is that this style of SIM card does NOT contain ANY personal information about you. Which, I guess some people on here already can see that it is.

I think everyone here has a total missunderstanding of Sprint's SIM cards. This is sort of an old thread for google to be leading me to. Help & Troubleshooting What is that 'UICC Unlock' menu item? It is in the software update are? What does it relate to in S4? General info in UICC.

Uicc Unlock From System Updates Menu