am i in the wrong for thinking my friend shouldnt be friends with someone

this will def just get lost in the void but if anyone does see this I hope they can help. I have this friend whos besties with like.., basically a "pdf file". and I don't know how to tell her I think she shouldn't be friends with him because every time I mention it shell get mad or act like she didn't here me.

so about this guy, he has in high school when we started dating long distance and I was in 6th grade. when then broke up when I was in middle school because he cheated on me with my friends little sister, she was probs in 5th or 6th at the time I'm not sure. I do know she is a few years younger than me though. I think its mad weird that she preaches about caring about her dear sister so much yet she's still pals with the guy that tried to get her to send gross pics.

I cant really drop her atm for other reasons but also id just feel bad for leaving her, what do I do :(