Take her out for an expensive meal, go back to your place, set up some kinky stuff like ropes and tie her to the bed then get a sharp knife, slowly carve off each one of her limbs then cut them into very small pieces.
Put all the small pieces of her limbs into a vegetable and chicken soup then knock of the persons door who cheated with her, hopefully his parent/s answer, tell them that you are your girlfriends brother and that your girlfriend wanted her secret lover to have the soup.
Wait about a week then send pictures through the door of the guy she was cheating with, with a letter titled with his name. When he opens there will be lots of pictures of her limbs, then add a picture of the limbs being cut up and put into the soup.
He probably wouldn't cheat with a girl after that, and well, your ex girlfriend wouldn't be able to cheat anymore. If you don't have a lot of money when you take her out for the expensive meal, order soup and take most of it back in a doggy bag.
You're welcome.
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