Would I be a bad person for not calling out these mean girls for bullying this guy?

I was playing an online game with a friend of mine that I’ve known for a couple years. We’ve never met in real life but we play games together. I just realized I think she’s kind of toxic. We were playing with one of her friends. I met him yesterday. She would jokingly call him dumb ass and idiot. Idk if he minded or not. I kind of tried calling her out a couple times. When she called him dumb ass I said that “well I doubt he’s a dumb ass”. At one point he said that she’s mean and she was like “because you’re an idiot lol”. I said “wow you are mean”. She was like “im only mean to people who have wronged me”. At one point we were telling eatchother our middle names and the guy friend was really embarrassed to tell us his but he eventually did. I told him he didn’t need to be embarrassed. There is another girl that is regularly in their party and he said that he would never tell her his middle name because she makes fun of him. Idk, it seems like this guy is getting bullied even though he could just not join their party. My question is, is it my responsibility to investigate this? I’d rather not join thier party again, but is it my responsibility to join their party when that other girl is in it just to catch her in the act and call her out? I mean, i don’t just join their party. I get invites. It would really be out of my comfort zone but it seems like it would be the right thing to do.