I know how it feels to be cheated on. The amount of anger and pain strikes you all in one, formidable blow only to leave you suffering the "aftershocks" of that pain for some time. How you once believed that this person would be yours forever, only to be proven a liar. You feel connived and deceived, like a fool completely blind to something, or that you failed to provide.
But despite all that she did, I personally found it reasonable to forgive my ex and treat her as a friend again. Never would I consider being with her again, however. For one, I have found another, and second, I personally do not feel I would be willing to give her my heart again if I could, I don't trust her with it. So, if I were the one with the choice, I would be friends, but not go any further than that.
But, Otebon is right. Just do what you are comfortable with. But keep this in mind: people make mistakes, big and small and anywhere in between. To allow a mistake to cloud your ability to perceive an honest apology is fallacy. This is not a direct statement to your thoughts and feelings, but simply a suggestion in that sense. Just be reasonable is all I'm saying, really, but never step outside your comfort zone in such a situation such as this.