Well it does have a lot more internal storage, different recovery, different bootloader, and hard key combo. The sound also sounds alot better than my other Razer Phone 2 that's sitting on my desk.Maybe he isn't setup with the a/b scheme?
Edit: Maybe the arter kernel isn't compatible with some hardware that's different? Causing kernel panic which results in bootloop? It's gotta be something simple though. Otherwise the att firmware wouldn't even boot at all.