Swedish Social Experiment - Domestic Abuse

Call the police and monitor from a distance so you can take the chance if it gets so bad you can't wait. At least then you have some backup coming.
But honestly without any special legal obligations I wouldn't be inclined to intervene unless it's physical and very one-sided, and then only to stop the fight. Who knows what led up to what you're seeing? If both parties seem happy to fight it out, why engage them alone? Wait for someone to be around to help you or for the number of combatants to come down. If someone's just being beaten, obviously go rescue them but there's no point jumping in to help either party until that party wants out of the fight (verbal or physical) in the first place.
If nothing else you have time to assess. Might save you from being surprised by something.

 

Just some loud talking. And she said only once Help. If I heard Help from her I would intervene otherwise no.

 

It was a long time ago and my accounts are not verbatim.

But note, people can be too terrified to call out. We are our brother's AND sister's keeper and we need to take the initiative sometimes - carefully, respectfully, intelligently.

 

Because they know they'll end up fighting both of them?

Women are not the way most white knights think.

Never intervene in that situation, she's with him for a reasons.

 

doing something or nothing

In England, for a population if about 60 million about 3 women are killed by current or ex-partners every two weeks.
You have to be as careful about doing nothing as about intervening.