I'm sorry for your friend's dilemma, Sabby, and you're a good friend for trying to help her through what is undoubtably a stressful time.
I would want more opinion if, say, I were on a jury trying to decide whether it was abuse or not. Is she almost 18 or is she 12? How old is he - is he 20 or 40? How long as he been an authority figure to her? Does HE see himself as an authority figure? When you say she "didn't try to stop the man", what does that mean? She didn't freak out and scream, or she didn't show any discomfort or hesitation?
I couldn't jump to a conclusion while knowing so little. I was also in similar situations (not really with authority figures, though) when I was a teenager and young adult, so I can indeed sympathize with the fear, guilt, and self-questioning.
Regardless, I'm sure that she is stressed and upset, and my heart goes out to her