Boyfriend talks to his female friend this way. Should I mind my business

I've been with boyfriend a few weeks now. I already know he has a childhood best friend who's a girl and she's cool. Most of my friends are guys. I was over his place and his sister was there. He was also on his laptop face timing with his friend and I was sitting next to him. She said she had something to tell him and held up a pregnancy test. He looks annoyed and said "Is it you know who's." She looks flustered and says "Yes. I know. You don't have to say it". He said to her "This is what I'm talking about you dumb bitch! You don't fucking think!" I was shocked with how he was talking to her. His sister said they just talk to each other that way. I think him calling her a bitch is disrespectful and basically making her being pregnant out to be a bad think is awful! I talked to my mom and she thinks I'm projecting what I've been through onto their friendship. I had an abusive ex who used to always call me a bitch and I also can't have biological kids which devastates me. My mom says she calls her friends dumb bitches all the time when they do something dumb and that even though getting pregnant would be a good thing for me, I don't know the situation so it may be bad for her. She says I don't know their friendship and we haven't been dating that long si I need to stay out if it.