<human>: Condense the text into a few key points

The elderly woman living upstairs my parents', I've heard her son yell at her numerous times, so loudly that I can sometimes hear him clearly shout "shut up" more than once and for her to stop bothering him. They're violent screams. Sometimes I even hear thumps, objects dropping and furniture being moved around.

I'm often at my parents and I hear him almost every time. It happened again a few nights ago, and once more, I froze. I felt like a coward for not doing anything at the moment and once the noise stopped.

I don't know how to handle this, I want to report it without causing further trouble for her... as in... if ever cops came to investigate and left, I'd be afraid of her son being even more abusive towards her if he thinks she's the one who called the authorities on him. I also don't want my parents to encounter any problems if he figures out we're the neighbors who did this.

What do I do? 

She has been living upstairs way before my parents and I moved in when I was a kid. It's always been her and her son. I've never had any real contacts with her except waving to each other when she's looking out her window. We did that for years, every day when I came back from school, she was there. Then I grew up, started working full-time, got a life and would come home very late.

Just tonight, I went over to have dinner with my parents, I saw her walking to the window and looking out for the first time in years... I waved at her, but she didn't do anything. Either she didn't see me or doesn't recognize me anymore. This made me a bit sad.
<bot>: Son screams at his elderly mother so often that my parents and I are scared for her. What do I do without any backlash against her and my parents as neighbors?