The people who hear such a thing happening, especially the men, have no excuse not just for not calling the police, but for not rushing out to stop this themselves.
On two occasions when my wife and I lived in an apartment I intervened on behalf of women who were being attacked (not raped, I'll note) by men. One of them was a total stranger to the girl who was beaten savagely and the attacker didn't stop roundhouse punching her until I drew down on him and ordered him to stop or else I would open fire. I meant it, too. My wife took the girl to our apartment while I held the SOB for the police (who did not show up for nearly an hour) and the girls blood remained on the stairs to our apartment for the next two years until we moved into our house.
The second occasion was when a man was beating the snot out of his wife and she was screaming for help. I grabbed a baseball bat from a couple of kids and went after him and when confronted he asked me," Are you gonna swing that bat?" as if I wouldn't.
I tapped it a couple times on the street and said, "Batter up." and then advanced on him at which point the coward ran.
Sadly, when the police showed up the woman refused to press charges against her husband.
I cannot fathom how a man could sit idly by while someone,
anyone, was being attacked and then have any respect for himself later on.
I'd rather die trying to help someone than to have to look at myself in the mirror and, worse, to have to look my wife in her eyes after losing her respect.