My friend got "cheated" on, do I tell her?

Hi everyone, 21F here. This situation is pretty long and complicated, so stay with me while I explain.

I’m part of a big friend group (7+ people), and we’ve all been friends since high school. A few years ago, two people from our group started dating -let’s call them Hannah (21F) and Max (21F). I’ve known Hannah since primary school, and she’s one of my closest friends. I met Max in high school, and we became close too. Another girl in our group, Sienna (20F), I have known since middle school. She has always been really close with Hannah and me.

Hannah and Max had a picture-perfect relationship. They were always seen and spent lots of time together, and while they could be a bit over-the-top in love, we were always genuinely happy for them.

After about two years into them dating, Max confided in me that their relationship had been struggling. He said they were fighting day and night, and that Hannah often tended to create problems out of nowhere. Around the same time, he started getting closer to Sienna and confiding in her too. I thought it was a little weird but didn’t see it as an issue—after all, we were all good friends.

But then, I started noticing some weird dynamics. The tension between Max and Hannah became obvious, especially during group hangouts, while Max and Sienna were really friendly with each other: They hung out a lot outside of group hangouts and during them too.

One day, Max hit me up, saying he needed to talk to me. When we met, he spilled everything: He and Hannah had fought so much that they didn’t speak for a week. During that time, he spent a lot of time with Sienna, going over to her place, late night drives, etc. He explained that one night at her place, they were watching a movie, and she started caressing him gently. He then fell asleep in her arms, cuddling with her in her bed.

I was shocked and immediately told him this was not okay. Even if it wasn’t "direct cheating," it was still incredibly disrespectful and hurtful to Hannah. He brushed it off, saying he would tell her eventually—but that Hannah wasn’t doing well, and this would "break her."

A few months have passed, and Hannah and Max did break up, though for other reasons. They even decided to stay friends. When I confronted Max if he ever told Hannah about what happened with Sienna, he denied it—saying he still couldn't bring himself to hurt her like that. He reassured me he'll tell her soon, but I really doubt it.

Now, I feel sick about the whole thing. I watch Hannah be around Max and Sienna and the thought that she's being left in the dark like this and lied to makes me furious and feel guilty. Max and Sienna are pretending like nothing ever happened and it disgusts me.

I’m stuck. If I tell Hannah now, she’ll be furious that I didn’t tell her immediately and will cut off the whole friend group. But if I don’t tell her, I feel like I’m keeping a cruel secret from her.

Would you tell her in my place? Or is it better to just let it go?