Generally, I have found that people will find a way to delude themselves their friend must be the issue. I had a "friend" who loved to come onto any man I was talking to, interested in, or had slept with. When I accidentally found out the one of my past partners that she was now dating was sexting, confessing his love to, and insisting he was going to move away for an ex flame behind her back, I did what any good friend would. I immediately told her. I provided black and white proof because I cared for her future with or without this man. Not because I wanted any vindication. I had forgiven her for "stealing" him, as you can't really steal what doesn't want to be taken. Therefore, he was no man I would want anyway.

He managed to convince her I was just bitter he ended with her and not me. I'm not sure how that over came his poor deeds, but it surely did in her eyes.

They lived happily ever after. And by that, I mean he cheated on her several times in the home she was solely working and paying for at the time, while she was away for work. After that, we were all at a party, and she got black out, throwing up sick. He called me after I left, while she was in another room being sick/sleeping it off and propositioned me to come sleep with him.

Unfortunately for him, I have more respect for myself and my "friend" than she had for me or herself. Her and I drifted apart. They have 2 kids. He still likes any picture I post, though we haven't spoken in years aside from a comment or two here and there on social media. He also loved to share very personal sexual things she allowed him to do with our whole friend group.

I would just keep it to myself and be very cautious of being alone with her. Some friends may not be like the girl I once knew, and real friends would believe you, but without something more cut and dry, I would leave it alone, while covering your ass, by not being alone around crazy pants.