The problem is that we don't know what was said, or how it was said. If the guy was yelling at the victim's wife, cussing her out, and when the victim came out he hears those words and sees his wife with a panicky look on her face...well I don't think many of us would stroll up and say "I say, old bean, what seems to be rustling your jimmies?"
Again, I've said above that I need more info before I can decide how I feel about it - the video tape (with no sound) makes it appear one way, but with all the data, it could paint a very different picture.
And regardless as to how we would or wouldn't react, the reality is the boyfriend was the one that escalated it to physical violence - up until that point, you had a woman who couldn't care less about other people and a dude that thinks it's his job to police other people instead of minding his business. Nothing from the video shows either of them doing anything that would cause the police to arrest them. When the boyfriend arrives, he immediately brings physical violence - the first obviously arrest-able offense - to the situation.
I'm not blaming anyone, yet. I'm not passing judgement, either. I'm simply saying that I don't know if the man should be charged with manslaughter or not, because we have little to go on.
As I've said above, I'd likely have responded in a similar manner - but I know (and have known) that escalating an argument to violence is adding a very unpredictable layer to the equation. One-punch killings aren't common, but they're not rare, either.