Second, there is the legal risk. It often isn't clear just what is really going on in such situations.
Very, VERY, VERY
true. Is the scruffy-looking guy holding a snubbie revolver on a guy in a business suit a robber or a plain-clothes detective making an arrest.
My advice would be as a couple of others have suggested. Stay out of it, get the heck out of Dodge, and call 911.