My friend who bought his Galaxy SIII the same day I bought my iPhone 5 has had repeated problems. Anecdotally his phone randomly reboots, randomly shuts down, all by itself as well and seems to be very erratic in its operation. He had buyers remorse two weeks into ownership. Which is too bad - the screen seems very nice for a droid phone.
I have the S3 for 6 months and never experienced a random reboot/shutdown. I just checked my phone status and it is now 1263 hrs (52 days) since last reboot which happened when I upgraded to jellybean firmware.
So, is your friend an imaginary friend? I rarely come across REAL S3 owners reporting random reboot/shutdown or erratic behaviour.