Supporting best friend

My best friend, who I’ve been close with for a decade, isn’t really known to make the best decisions. She’s cheated on multiple people, had sex with people in relationships, and stirred up drama for a long time.

I don’t know if I should support her as her best friend—she’s happy, so I should be happy for her—but the fact that she does these things REALLY bothers me. She recently had sex with a guy who has a girlfriend (she says she didn’t know at the time), and she doesn’t seem to think it’s a big deal. She refuses to tell the girl because it’s just a “consequence of dating someone who cheats and she’ll find out sooner than later because he’ll do it with someone else.” I told her that telling the girl is the right thing to do, but she disagrees. (I’m also good friends with the girlfriend.)

I’m wondering what you all would do in this situation. The girlfriend is such a sweetheart and deserves better, but I don’t want to cause a huge fight with my best friend by constantly bringing it up. I’m not sure if it’s my place to tell the girlfriend, but I really want to abide by girl code and prevent her from finding out in a shitty way.