911 advice needed — tell my ex bestie she got cheated on?

Long story short — I recently found out something terrible about my ex best friend.. (we are not on talking terms because she’s stubborn and doesn’t like to admit when she’s wrong) however I still love and care for her. If I reached out, we would talk but I am tired of being the “bigger person” considering I reached out previously and was ignored.

What I found out: her finance cheated on her. With a guy!!!

Yes it’s confirmed.. other good / close friends know and have yet to tell her. They don’t want to hurt her. They said the guy is threatening them. I feel like she deserves to know, no matter what. Others are saying they don’t want to ruin the relationship because they both are very happy and it was a drunken mistake?! The guy is also denying it. They are saying she wouldn’t believe anyone anyway or maybe she knows already (that he’s bi / gay) however even if she has a feeling of that.. there’s no way she’s okay with the cheating. Even if she doesn’t believe me. She can ask others or at least do whatever she wants with the information? I just can’t believe no one has said anything.

I’m torn because although she wasn’t always the best to me, I would want someone to tell me. Even if she never talks to me again, she deserves to know.

What do you guys think?

Ps I just found out — however this happened 3 months ago