Samsung introduced a new way of locking down the bootloader in 4.3+. On the Verizon S3, I believe you can still root but cannot install a non-Touchwiz ROM (like Cyanogenmod). I'm not exactly sure since I rooted and put a custom ROM on before 4.3 came out, so I haven't needed to worry about it.
The S4 is a newer device with a larger community, so perhaps the devs have figured a way around it. But it's something to consider.
Check out XDA for more information. Here's a
Sprint S3 thread about Knox.
Edit: sorry I think I misunderstood your question. If he takes his phone in and gets a new one back, the new one will be unrooted and on Android 4.3, which has a bunch of extra features that Samsung added to make it more difficult to root and ROM. The developers may have figured out ways around it, but just wanted to give him a heads up.
Last edited by dukemagic; 02-06-2014 at 12:23 PM.