in need of advice

so i've got this friend right?
and like many of my friends she comes to me with her relationship problems, and i, as usual set up a time to go out to eat, so we can discuss this.
seven o clock rolls around in the big town of bagley and i go to where i was meeting my friend, it's there that she drops a bomb on me.
she's cheated on her boyfriend, the one she sees "forever" with.
now dont get me wrong, i was very surprised to hear this from my friend,
first off because she's never cheated in her life, and secondly because she seemed to actually want to spend the rest of her life with this guy.
she explains to me how they were out at a party, and then a group of their friends procceded to go back to his best friends apartment where they were spending the night. while there they drank some more and things started getting slightly uncontrolable. as i see it, people were dancing, music was playing, and her boyfriend was out on the patio smoking when this all occured. lately my friend has been feeling a little bit suffocated by her boyfriend and she was explaining this to one of his roomates. talking lead to longer talking and eventually they ended up kissing. when suddenly the conscience that my friend has, kicked in, she quickly pulled away, apologized and ran into the bathroom.
and that is where she spent the night.
in the morning she didnt remember what she had done, one of her girlfriends had woke her up and explained to her what she saw, and what her boyfriend had seen. she knew things weren't going to be ok after that, so they sat and talked. aproaching her boyfriend she tried to explain her motives, but something just seemed wrong. she admitted to her mistake, and promised it would never happen again. she explained that she knew what she did wasn't right, and that it was not smart on her part.
after many tears shed, spilling her guts, and mascara dripping down her face, the only thing he had to say was "i cant believe you" he then procceded to get up, walk out of the apartment and drive off leaving her there with no ride.
hours went by and she patiently waited for him to come home,
she tried calling him,
over and over again,
she left him voice messages,
text messages,
anything.
and still he ignored her. eventually they started talking, but things were different and both understood that. so fast forward two weeks, when my friend and i are eating. she explains to me the rollercoaster of their relationship since then.
things were bad, at first,
she gave him a week, and things seemed to be just like normal.
and then he snapped. constantly calling her, arguing with her over nothing, accusing her of cheating, getting angry when she wouldnt pick up when he called, things to that effect. she explained to him that she was busy, for she was, her life had suddenly gotten busy, she had rehersals for her play every day, and after practice work until closing every night, as well as being one of two captains for the football cheerleading team they had been conducting try-outs, end of the semester had begun, family problems were errupting, her best friend was having family problems as well, and now her boyfriend was blowing up at her every minute they talked. it got to the point where she felt like everytime they talked it was to argue.
he never wanted to see her, she tried to see him, and when she did it was akward.
and so this relationship has gone, my friend is currently in a dilemma. she doesn't know how much more of this relationship she can take, she still believes that she is in love with him, and regrets kissing that boy every single day, but she thinks her own guilt should be enough of a consequence for her. considering she had been the perfect girlfriend for the past three months. when she came to me i wasnt sure what to tell her, i told her to do what makes her happy. and now we're coming to you. what do you think my friend should do? and do you think her boyfriend overreacted?