How do I move forward when my younger friend chose to stay close with my older ex, even after everything he did?

TL;DR:

I’m 15F. About two years ago I briefly dated an older guy (18M) who became inappropriate and caused a lot of drama.

About a year ago, I met my younger friend (13F) and we became inseparable. I trusted her and told her everything about my ex and why he’s a bad person.

After I met her, she became best friends with him anyway. She later dated someone else, but her attachment to him caused problems in that relationship and her ex eventually broke up with her because of it.

Even knowing everything, she told me she can be his friend “even if he’s my ex.” I care about her a lot because she’s young and vulnerable, but it’s hard to move forward and protect my peace while she’s choosing to stay close with someone who treated both of us badly.

How do I move on and handle the stress without creating drama?