I would confront her, tell her that she cheated, prove my point, then break up with her and never see her again. Because if a person cheated once, they are more likely to do it again. Then move on with my life.
People probably cheat on each other because of the problems and situations that strained the relationship. Also, some personality traits and beliefs may change a person's viewpoint of the relationship. There are many reasons why people cheat, its not a very simple subject. Most likely due to problems with the relationship, like not enough attention, too much attention to details, trust issues brought on by childhood repression. Damn I am getting all Freudian on this.