What should I do when someone is harassing my neighbor by knocking and shouting at 2 am?

Happened just 30 minutes ago at the Park On Cambridge complex in the Astrodome area near UT Health School of Dentistry. A man started knocking at my neighbor's door and shouting, the answer was pretty hostile by the neighbor as well.

Everyone woke up, I can literally hear my neighbors waking up.

I hid in the bathroom for safety, but it was difficult to ignore that everything happened literally right in front of my bedroom window (Couldn't see because the blinds were shut).

Anyway, after reciting bible verses and Psalms for protection (I was terrified) I came back to the bedroom and was still able to hear the knocker's voice, albeit distant. Took a peak through my blinds and it seemed like the neighbor right next to the one whose door was being knocked has actually started talking to the harasser and has actually managed to take him away from our apartments and into a more open area.

I am still shaken by this, and to tell you the truth I am scared. The gentleman who took the harasser away was literally in his underwear, he didn't have time to get dressed.

What do I do in a situation like this should it repeat itself? I was scared something worse might take place, and my neighbor saved the day just now but he's not always there.

And why didn't the neighbor who was being harassed call HPD? Seriously?

Anyway, this night was uncomfortable and scary. I didn't feel safe in my own apartment because of the windows which were very close to the incident.

Any advice? Please help.