If he knew it was a gun, why would he shake it out of the sock into his hand, and not take it out carefully?
If he didn’t know what was in the sock, why would he shake it out of the sock into his hand, and not onto a table or floor?
Not that there couldn’t have been an accidental discharge either way, but I just don’t buy his story. He shakes the sock (either knowing it was a gun, or not knowing what it was), into his hand, it lands perfectly aimed at his coworker, goes off, and hits her in a vital area causing her death. While technically possible, I have serious doubts that it actually happened this way.
I saw a show that had a perp robbing a place with a gun - he knew it was a gun and that it was loaded. When he was sure there would be no resistance, he got careless and played with the gun a bit - it fired and hit the clerk. It was loose in his hand so it rotated back and the trigger reactivated as it was pointing at his own head. The store video shows a the prelude, then it is BANG-BANG, with clerk bleeding from his side and perp out of site and dead from blowing his own brains out.
When someone who doesn't have experience handling weapons, as I assume is the case with the "sock accident", I have no trouble picturing a dozen ways it could have gone bad.