At this point you don't know anything about what happened, other than what your GF's friend told you, and who knows how valid she is as a source. You'd be stupid to try and fight someone, especially if you don't know the full story. Even if he did rape her, it's out of your hands completely. Your only option is to talk to Natasha about this. Tell her what Cherry told you, and see what she says. If she casts it off and simply says she was too drunk to remember what happened, then it's more likely that she cheated on you. But you won't know unless you talk to her. It's worth it to say that you should be 100% sensitive to Natasha before you make any assumptions of what happened. If she was raped, she is surely traumatized over it, and if she feels like you are accusing her of cheating when she was raped by someone, then that could end up really badly for both of you -- her especially. When someone is taken advantage of, it f&cks them up in a lot of different ways, and everyone reacts to it differently. There is no "right" way to handle it, because everyone experiences trauma in their own way. I don't want to suggest that she was raped, because you don't have enough details to know what happened. You need to talk to her about this. The sooner, the better.
"Caring is not an advantage."