I fear she will only get hurt

I know this is long but please bear with me. I need as much help as possible.

Hi im 14 and my cousin is 16. Although I have never met her, we've talked online for 2 years and we tell each other every detail of our lives and help each other put with everything. Basically we keep it 100 and would never tell our secrets. its always between us and I thought nothing would break us up. Until...

Well, let's just say Ive seen and helped her through a bad breakup with a boy. Now she's with another one and im worried about her. Last school year, she lost her virginity to him in the school closet to him Then more problems arose when he said he didn't really want them to be together and he "had needs" over the summer. She suggested fwb with him and said he wouldn't sleep with anyone else because "he knows it will hurt me". Now that school started back, he pretends he doesn't know her at school and has talked bad about her. We have gotten into so many fights about it I just say "ok" whenever somethings happens between them. Now she basically tells me she doesn't give a fuck about my warnings and will do whatever she pleases (she doesn't have much freedom at home). I want to help her but she's hard headed and refuses to listen. I don't wanna see my naive cousin hurt because she will give unlimited chances. They're talking about an actual relationship and she turned down a good boy for him. Someone please help me warn her.

All of the information given was told to me by her. The only reason I'm saying this is because she has a soft heart and is already confused in every way about relationships.

Please help I just want to save her poor heart.