What should I do about my dear friend?

My friend, CJ's (short for Carla Jade) mom and boyfriend had been together for three years now and they REALLY love each other. CJ's father abandoned her and her mother when she was about 8, so she was without her dad for about 7 years. She's 15 now. But anyways, CJ's mom's boyfriend proposed to her and her mom excepted. CJ is really MAD about their engagement. She want's them to break up. Her mom's hoyfriend's a really nice man. He never did anything to hurt her or her mom. I don't know why CJ want's them to break up. She started to cut herself (no joke) since then. What should I do?