I'm going to make my best friends girlfriend cheat on him to get him out of their abusive relationship.
Why do you want to inflict such emotional pain?
I'm simply giving him an opportunity to be free
While it's good that you want to support your friend and get him out of a relationship that is abusive, it is wrong to do so by getting his abusive girlfriend to cheat on him. You should try instead to support your friend so that he has self-confidence necessary to leave his abusive relationship. People remain stuck in abusive relationships when they think they can't do any better so you need to give your friend the self-worth to stand up for himself.
Nah, I want to torment him with the pain of witnessing his love interest cheating on him because it is the only way that he will learn.
I know that you want what is best for your friend but setting him up to experience the pain of having a cheating partner is wrong. Sit down and talk with your friend and give him the chance to understand the position he is in rather than trying to manipulate him with suffering.