Friends boyfriend treats them and their son like garbage

Just... the title. My friend and their boyfriend have a kid, theyre not married (yet) and he treats them like crap. He blames them for a lot of things and says theyre not doing enough even though they are working a full time job and hes not working at all (despite complaining about how he needs to work and be the provider). He does the bare minimum for their kid, and gets pissed off easily when the kid is having a hard time. This results in the kid being smacked or kicked somewhere. Tonight everyone is trying to get ready to leave, so the kid is throwing a fit. Instead of redirecting his kid he picks him up like luggage and threatens something if he doesnt stop kicking. Lets the kid go, he runs to the door, I pick him up, and bf decides to blow on the back of his head, to which the kid raises his hand and dad hits him in the stomach. I say "hey" and give him a look and he starts going off about it's his kid, be mad about it, and to learn my place. He got mad at his kid for having the same reflex that he would have and the kid gets hit for it. I just dont agree with it. I dont want to be this guys friend but its like I dont have a choice because hes with my friend. I hate the way he treats them both but if I say anything im the worst kind of person.