Cheaters

I have dealt with my share of cheaters and I don't get it. If you are going to cheat on someone you are in a relationship with why are you in that relationship. Recently, I have been dealing with (inside my group of friends) this girl who cheated on her boyfriend with his best friend and got a ticket for it. She is not the smartest person in the world, so she passed around the ticket at school. Then she had the audacity to text me the next day asking me if I was mad at her for what she did. I am like "what in the world?" Her boyfriend is one of my friends, so of course I was going to be mad. I have lost all respect for her and I am not going to be her friend, (the truth is I didnt like her very much to begin with). There is just a line you don't cross and she has.