He's 19 - did he leave his girlfriend for a 15-year-old?

In the end, a man leaves a woman because he's unhappy for some reason. It doesn't make him a bad person, and it doesn't make this cousin a bad person either. Trying to figure out how Carla played into it will do nothing but stress everyone out. Instead your friend should try to figure out what she learned from this relationship that will help her later in life and move on. The end.