Should I tell her or just let things happen?

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My best friend's boyfriend is cheating on her and she has no idea, I know she doesn't because she's talking about getting married to him. I've known for a while about his indiscretion and I told my friend to be careful, but my warnings go unnoticed; maybe it's because she knows I don't like the guy and he doesn't like me. They've been together over a year, he tells her he loves her and wants a baby by her. Now they are working on having that baby. His ex-girlfriend, who he was dating while he was with my best friend, is a friend of mine and she doesn't know my best friend is now dating him, i know she doesn't because of the reason they broke up. Anyhow the ex told me that he asking her to get back with him, that he misses her, and that she was the only one for him and he let her go. Now I want to tell my best friend everything I know but I'm not sure how to because they're a lot more to the story that i know will truly hurt her. I dont want to see her get hurt again, i love her just like if she were my sister. Should I tell her/ how should I tell her or should I just let things happen and hope that something change and some good happens.