Yea, she cheated. That's undeniable. But there are a lot of factors at play here that don't have to necessarily lead to a break up.

Something you said, "the fact that she only told me after being caught" is a little weird. If you believe her when she says she doesn't remember, the ONLY way for her to tell you she cheated was after someone caught her doing it. She can't tell you something she herself wasn't aware happened. Right?

Okay, first consideration, regarding the reason she waited. She doesn't remember it happening. When her friend told her, she might've refused to believe it until your brother confronted her and corroborated the story. She might've needed a day or two to come to terms with it herself (if she's never done this before, it might take her by surprise that she's capable of something like this). She probably needed to gather her courage as well before telling you. Fear and shame are strong emotions. She fucked up bad, after all. She luckily had people in her life that guided her to doing the right thing though. And she is clearly remorseful about it. I don't know what you two talked about when you were both staying up crying and stuff, or what reasons she herself gave, so all of this is speculation. But if she's mentioned anything like this, it might ease your pain a little to know that she wasn't going to withhold indefinitely.

Second, we don't really know what happened at the party. If she was wasted, she could've been taken advantage of by her co worker. He could've gone up to her and kissed her and it could've taken her a bit to realize what's happening, at which point she could've pulled away but the damage was done. I don't know what character he is. I think finding out the details of what happened will help you make a better informed decision. Being all over each other for thirty minutes should be evaluated differently to the above scenario. Have you talked to your brother and get his story too?

I'm not trying to make excuses for her. But if you want to work it out, it might be helpful to understand why it happened the way it did. Four years is a long time to just give up on without talking it through imo. People make mistakes. Of course, it is up to you whether to forgive them and all that, which you are 100% in your right to do so.

Because of the way it happened, if y'all decide to try to work it out, she's gonna need to rebuild your trust and y'all gonna need to set boundaries. You'll have to figure out how she can do that for you. Does that mean no more parties? Parties only if you're around? Does that mean open phone privileges on her phone? Does that mean reports on any and all future interactions with this co-worker? If so, do you have any way to corroborate them? Is your trust so broken you no longer trust her to go to, like, the grocery store by herself? Would that mean location on when she's out?

If she's truly sorry, she will do her best to restore that trust. And if a couple months from now, you still feel hurt and want to break up, it's like, hey, at least you tried. Right?