Update - so it appears that the Bluetooth may be the issue here. I had the EU turn it off (in Manage Connections), and the device was fine.
Once he turned it back on, it started rebooting/crashing again. It seems to crash predominantly in his car, and per the earlier reply, I checked and the battery strenght is at 92%, so that doesn't appear to be involve.
It doesn't appear and one can uninstall "Bluetooth" (it's not listed under applications), so I may either just re-install the OS, or contact T-Mobile and see what they say.