How can I inform a girl that the guy she was involved with was cheating on her with me and is not a good person?

I've (F26) been involved with a guy (M36) who has a situationship (F27) in another country. For the first couple of months of dating, he repeatedly claimed he would end things with her because he didn't have feelings for her and she was more invested. However, he lied about several things, and I found her Instagram. She has pinned photos of them that look very much like a couple, which contradicted his claims. After a big fight, he told me he ended things with her, but we didn’t talk for two weeks before we started seeing each other again. Recently, he went back to the US, and I saw on his story that she shared a picture with him sitting on his lap, captioned by her friend: "stop being happy 🤮." This made me question everything. I’ve been contemplating whether to inform her about what I know and to end things with him for good. I wrote her a detailed message explaining the entire situation, including screenshots of him arranging dates with me while he was with her and his intentions to end things from the start. I'm unsure how to approach contacting her, considering she blocked me after viewing her story during our fallout. He later claimed he broke up with her around that time. I would need to reach her through Facebook Messenger since we're not friends, but I’m worried she won't believe me or that he has misled her about the situation. I want her to know the truth, especially since I've moved on for good and don't want any further involvement with him. I even noticed he was chatting with other girls and still talking to her within 24 hours while I was at his place. I didn't confront him about this at the time, though he did ask me to be exclusive that night, which I declined due to my mistrust. He has been disrespectful to both of us, and I fear he never truly ended things with her or was in a real relationship. I considered reaching out to one of her friends to see if they could tell her instead, but they don't accept message requests from people they don’t follow on Instagram. Any advice would be greatly appreciated. I just want to ensure she knows the truth and that he doesn’t get away with treating people poorly.