My (22F) drunk friend (23M) crossed a line at a party… should I tell his girlfriend?

A couple weeks ago, my friends and I were at a party in another city. We all slept over at a friend’s house. One of our friends (let’s call him Thomas ) got completely wasted. His girlfriend , who is also a part of our friend group, didn’t attend the party.

Things were fun at first, we were dancing, laughing, until Thomas started dancing too closely with me. It got weird fast. He kept trying to close the space between us, and at one point, physically tried to turn me around so my butt would be against his crotch. I told him to back off, and he did… for a while.

Later in the night, he came back and slapped my ass while mumbling something I didn’t catch. The room went silent, and that basically killed the party. Everyone saw. It was awful.

The next morning, Thomas swore he remembered nothing. He apologized repeatedly and seemed genuinely ashamed. I accepted his apology in the moment, but I still feel very uncomfortable about what happened.

Now here’s where things get tricky: Thomas said he was going to tell his girlfriend about it. But one of the guys in our group convinced him not to, he told him it would only cause unnecessary drama and that she might break up with him over it, which might split the group since we all like her very much.

So now she doesn’t know. But we all do. And she’s still showing up to hangouts, laughing, talking to everyone, including me, with zero idea that this happened. It makes me feel gross, like I’m holding a secret that would shatter her trust in all of us if she ever found out. Especially me, since I was directly involved.

I’ve been feeling pressured into going along with this unspoken “pact” to protect Thomas. But I didn’t agree to this, and I don’t think it’s right. Still, I’m torn. Would I be overstepping by telling her? It wasn’t about her. But at the same time, it kind of was and if I were in her shoes, I’d want to know.

What would you do if you were me?

TL;DR:
My drunk friend made me super uncomfortable at a party: dancing too close, then later slapping my ass. He doesn’t remember it, apologized, and wanted to tell his girlfriend (also our friend). But another guy told him not to, and now we’re all keeping it from her. I feel pressured to stay silent but hate being part of this. Should I tell her, or is it not my place?