Friend issues please help?

Ok, so I have a friend and she has been admiring this guy from a far, I guess you could say, for about 3 years now. She finally got the guts up to talk to him and they kind of hit it off. They've been "talking" for a few months now and he has made her cry a few times, he has led her on that he wants a relationship and then let her down by saying that he has no interest in her, then flirt and tell her she's gorgeous again the next. See, he has low self-esteem. His step brother is in the same grade and is probably the most popular guy in the school. He is cute, and nice, he's just not his brother. So recently, she has been messing with him and it started off her being like "lets see how he likes being led on." Now it has progressed to raunchy messages, I guess sexting? No pics but they talk like they're gonna do it soon, and she'll tell me about it and stuff. I freak of course, as a best friend because she has not had slutty behavior in the past, he is bringing this out of her and any outsider can see that they're relationship is not healthy. SO she reassures me that she would never do it with him, because she doesn't know him that well. Then she asks me questions like "what if we did?" and say stuff like "but he says he'll go easy on me..." (she is a virgin). Usually when she proposes things like this surrounding an idea it means that she is actually gonna go through with it. So now, it looks to me like it is gonna happen. I can see whats going to come of this already: girl gives it up to boy, boy never calls back (Because he already told her he did not want a relationship), fights arise, girl is a blubbering mess to best friend (me), whom she expects to pick up the pieces, and I do that by blowing his brains out (Kidding!). Kidding about the last step. Anyways. She is not dumb, she is a very intellectual and amazing young woman. She needs better than this. Before she met him we both shared the idea to be in love before losing our virginities and now it looks like she's gonna give it up to a guy she's shared a single trip to the park with, walking after one class with, and sharing text messages for 3 months. Maybe she needs to get hurt so she'll learn and it won't happen again, but I want to prevent that to the best of my abilities. She also doesn't spend much time with him, actually none now that schools out. Just texting. So I am very afraid that if she tries to go somewhere with him and reveals that she was just teasing him, she could get raped. Please help me knock some sense into my best friend before she hurts herself so blindly. Last thing, she just turned 16, he is 17. I am 16 (if it matters). Please help!! I don't know what else to say to her!