If he unregisters the device, then gives it to you... you will not be able to read the material purchased from Amazon. Public Domain and other non-drmed files will still be readable.
It's not the device specifically, but a combination of the device and the user's account.
For instance, my wife and I both have Kindles (Mine is a DX, her's is a Kindle 2) and we can not read each other's books on our respective Kindles. If I were to de-register my kindle, the books I've bought from Amazon remain with my Amazon account so that I could put them on a new device later on, but she can't read them. Period.
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