My first impression was this: people make mistakes. As most people know, I used to be in an abusive relationship, and even after I was out of the relationship, I cheated on my boyfriend with the guy a couple times. Half was rape and manipulation, but it was my fault for putting myself in the situation...
How was her relationship with this guy before you? I most certainly understand your dilemma. My boyfriend and I have been there, but since nothing has happened since, things have gotten to be wonderful between us. Did you read her e-mail intentionally? Or was it just on the screen? You said she hid what she did for a month, do you think she would have told you had it not been of her friend?
Sorry for all the questions, but maybe I can be more help with some answers [:
