Begin to screw with them by acting ignorant, like it never happened. Then start to throw in little hints and such, like forgetting their names, how old they are, important details, etc. Interrupt them whenever they speak, but still act like it’s an accident, and let them continue. (I would advise practicing this beforehand) With a group of friends, tell them to only mouth the backstabber’s name and details when he’s present. If they do not know he backstabbed you, play it off like it would be a joke, however it would be much simpler if you were to let them know. After a couple weeks/months of this treatment, begin to pretend to completely forget the friend is even there. Say things like “Who are you? Oh yeah” and other similar lines. Make them think they are turning invisible. A month or two later, completely ignore them. If you bump into them, keep walking as if nothing’s there. If they talk to you, don’t respond even in the slightest. If he lives with you, “forget” to feed him/her, and do the above. Winning mentally is always satisfying, and there’s hardly any guilt to go with it.