Yes he can. Even in "At Will" states you can't fire for reason which is discriminatory, retaliatory or otherwise illegal. This particular case would be under wrongful termination law: violation of public policy.
Both of you are correct.
Bighorn is correct in that if he can prove his case, "at will" clauses don't indemnify employers from federal law.
Semp is correct in that most employers will officially state a 'BS' reason for firing someone that is not related to the real reason of the firing, and "at will" employment means the set of excuses that can be used is larger.
Can someone explain what "rooting a phone" is/does?
Most devices purchased from a phone carrier are 'locked'. IE, in order to provide the experience the carrier intended, they impose artificial restrictions (via software) on how the user can interact with the device (including what software can be installed and what aspects of the hardware the software can use). Rooting ones device allows for the owner of the device to utilize the device in any fashion they choose, install any application, access any aspect of the hardware, etc...