well - from the basic description doesn't sound like most benthic species, I know of.
But the fact that it was 2-4' gray & stocky - gives me great concern.
sounds like it may most likely a type of requiem - which could range from a adult/large sub-adult sharpnose, or bonnethead, to possible one of the larger requiems, like Finetooths, Blacktips(not BT reefs), or Whitetip reefs.
While requiems like the Sharpnose & bonnethead are generally stocky - they are still generally stockier(for their size) than the more well known & aquaria friendly species - like smoothounds.
Additional details may help to clear up the mystery.
I don't really want to ask what size of tank your friend had it in.
I would hope a large pool of several thousands of gallons