Only she can really explain to you what was going on in her head why she didn't stop him.
Can she be trusted? No. At least, not in the short term and you're going to have to treat her as such. She is also going to have to agree that you will treat her as if you do not trust her.
Either she agrees to a relationship with complete transparency (And this goes both ways.) or the relationship ends for your own personal sake.
People make mistakes, cheating once does not make a habit but it does make for needing to watch out for your own interests until you're sure she's completely on the up and up.
Which means she has to understand that you're going to be paranoid, and upset, and angry, and jealous, and that you're going to do things like snoop in her e-mail, and check her phone out periodically.
And, it's quite possible that she felt powerless to say no and/or stop him. That it was effectively a rape situation and that she was truly too scared to say no, stop, or anything like that.
I'm not saying either is the case, but there's a lot between here and there that you don't know, and what she may really need from you right now is support and help to find counseling. She's going to be your best source of information, and we'll certainly try to help you translate it as much as possible. But the words have to come from her.
But, work at getting all that stress and energy out of your body and keeping it there. Playing drums, running, sports, anything to keep active.
Oh, and you should never feel unable to talk to your parents when you're having relationship problems.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."