It's mean to dump someone for someone else.
Melanie's boyfriend just introduced her to his roommate, who is really good looking but single.
Melanie wants to help her boyfriend's roommate find a girlfriend.
Melanie introduces her boyfriend's roommate to one of her good friends who she thinks will make a good match.
Melanie breaks up with her boyfriend, so she can ask out her boyfriend's roommate.
Melanie's boyfriend is happy that Melanie helped his roommate and his feelings of love and loyalty for her increase.
Melanie's boyfriend is heartbroken and feels betrayed by Melanie and jealous of his roommate.