Look, if I saw a woman who was in immediate risk of danger with a man (partner or otherwise) who was threatening her, raising his voice towards her, actually being physical, you can be damn sure I'd do something. I'd call the police or alert security. I wouldn't wait for him to go to the toilet and discreetly ask if she is OK. So that's not the scenario being discussed.
I quite often observe situations I'm not sure about and hang around a bit to keep an eye on it and make an assessment - like older men taking an interest in a much younger woman at the pub or a man who has approached a woman at a cashpoint or just a man 'loitering' around women and looking suspicious. I've offered company and support.
I've checked on women in toilets who've just got away from the bloke who is well known for targeting young women in the pub to check she's OK.
If I had a friend I was concerned about I would (and have), when we were alone, check in on her. Because she knows me and knows she can trust me.
My partner and I invited a woman to come and sit with us on a night out. She was alone watching her partner's band play and she was being circled by predators. We didn't even talk beyond introductions amd polite small talk but at least she wasn't sitting on her own.
I let those women know. Because their risk is neither increased or decreased by that. And sometimes those men are deterred by the visibility.
And I've had people do the same for me.
But being concerned about an actual couple who are complete strangers to me just because she is looking a bit sad or worried is very different. That woman has to go home with that partner. Her life is tied up with his. Their might be children. She might not have access to money. She is not gong to leave because a stranger asked if she was OK.
The man who accompanied me of at the station when I was harassed on the train? Very much welcomed that!
No one is saying never step in under any circumstances just that believing it I'd always the right thing to do and that the potential benefits always outweigh the risks, isn't accurate.