It's good your friend is ok.
Would others have done the same in the situation? I ask this not from any sort of legal perspective, but from a personal safety standpoint. I might've been too afraid that the dude would've unloaded his shotgun if I had tried anything. It almost seems that the only thing that saved him was that the perp dropped the shotgun as he fell back. At a range close enough for me to touch the barrel, a shotgun blast would've been devastating. Maybe it wasn't loaded, but you can't know that at the time.
Then again, I suppose you can't whether or not the perp would just shoot you once he gets your wallet anyway...it would've been a tough call.