I can't stand this anymore! And I can't help!

I am at my gf’s apartment (she is out right now). There’s a guy upstairs shouting (and possibly beating) his 9-year-old son. From what she says, he does it all the time. She is not meant to be living here, so she can’t go to the police with this, and neither can I (I don’t speak Polish to start with). Given my own recent experience, I really want to just go up there and beat the shit out of him. This has been going on forever and none of the other neighbours seems to care!