First, I am NOT a lawyer, nor do I work for an insurance company. However, I do know a little.
Workman's Comp will usually only pay off if the injury was sustained "in the execution of the employee's normal duties." Unless "maintain order" is in the Employee Manual, breaking up domestic fights is clearly not part of a McDonald's employee's job description.
I think he's SOL.
Secondly, what he did was admirable on the surface, but really dumb deep down. He walked into a situation he knew nothing about. "Third person defense" is a clause in most community laws, but is hardly a license to go out and play hero. In fact, as seen here, it can be extremely dangerous. Although I carry, and the "third party" defense is in our laws here in SC, I would have NEVER interfered in that instance. My actions would have been to call 911, and get good descriptions of the combatants and a good report of what happened for the LEOs when they showed up.
You're always on shaky ground concerning safety and legality when you step into a situation that doesn't initially involve you. The man slapped the woman. I'm assuming a lot here. I assume no weapon was shown at that time. I assume the woman wasn't being held and could have gotten away from her assailant. From the report, the woman was not "in imminent fear of her life or grave bodily injury." Absolutely NO cause to interfere, except for the sake of heroics. That judge, IMO, is an idiot. His praise for the employee will only encourage others to act in a similar manner, possibly getting them into the same trouble this young man is in now.
We are not vigilantes, we are not LEOs. We carry a weapon for only one reason - to defend our life or the life of a loved one. Period.
Don't get me wrong, I DO understand what motivated this guy. However, in this instance, his heart was in the right place, but his mind wasn't.