Please.. You want to hear what you think we should tell you or do you want to know what we're actually thinking based on what you've shared? I don't give a flying shit if you started out casual or not. Respect her and if you don't then leave her alone to find someone who loves her and has come to terms with her past. If you're going to make it past the first night then you should know what makes you want to run for the hills and stay the hell away from her if what she did is something you know you can't get past.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion