I witnessed my neighbor cheating and did nothing about it.

I (30M) am living in this new apartment since last year. Everything is normal since I have been here. My neighbors are actually quite nice, however I know no one really personally (typical apartment life). There is a couple living next door, the guy is very nice guy, he helped me a little when I came, I really like him. His wife is also always nice and smiling. They are both maybe 2-3 years younger than me. Last week, very late at the night, maybe around 11 PM, I figured out I have forgotten something in my car, it was actually a quite important document, I headed through the door (pissed), as soon as I wanted to open the door I heard my neighbors, well as a quite introverted guy I preferred to stay inside so they could go inside. I saw through my door's camera, it was the wife with a completely new guy, at first I told myself, well maybe he is just a relative or something. Later that night when I wanted to sleep, I have heard them having sex very loud. I have never ever heard anyone since last year. Our walls are thick AF. She said the dirtiest things possible and moaned loudly. I got hard. Yesterday I saw the man with her and she acted like nothing happened lol