How do I get this girl to break up with this guy that is not right. For her?

See my best friend and a guy named tres are going out and the last time they went out everyone made fun of them but everyone says they are not meant for each other they fight alot and a bunch of ppl think they r going to have a kid before they graduate and I kno tht he will either rape or hurt her to have a kid because I kno this girl very well I mean she is sister to me I mean she means the world to me and I really don't and wouldn't like if something likes this happen to her he trys to be cool and kiss her out in public but they have never kissed all they have done is held hands what should I do