The article isn't clear, but my assumption is that the 'hero' went hands on with the man to try to stop him. My thinking about if he had a gun, he could have kept some distance from the beater and the beatee.
In a situation like this man faced, I would probably verbally challenge the beater and go from there. I would definitely not be in close range with him though. Given a parking lot probably a car between us would be good. If the man and woman stop and agree to leave together, not much I can do. I'll still make a police report though. Anything else would have to go with what was presented.