Okay that changes things completely. I do not want to use triggering terms, but based on what you report here/what she remembers happening with C, that's a very toxic relationship. I think you are right to dissuade her from hanging out with C. Even if nothing is going on (which it sounds like nothing is), feeling sick around him is actually a trauma response. This person is not good for her. I understand as a friend it can be a bit delicate to try and navigate something like this, and I am sorry you are in that position. You obviously care and are a good friend. Unfortunately all you can do is voice your concerns. She has to make her own decision. I hope for her sake she decides to stop seeing C, and I hope you're able to practice good self-care whilst trying to support your friend, because something like this can be emotionally exhausting. I know it's hard, but don't be afraid to step back a bit if you need to. You should be your first priority, you know?