I am sorry for appearing too coy about your situation, but I needed to know that first. Now, let me reassure you that it IS rape!! There is NO such thing as "consenual" sex with a MINOR!!! He knows that. He should have stopped immediately after your friend starting saying no. The fact that he didn't puts him in a very different light. Yes, her parents could and SHOULD go after him. She needs to know that she didn't ask for this. While the appearance seems to be that she ok'd it, she changed her mind and that made it rape.
He shouldn't be having sex with a 16 year old anyway. She
really needs to tell her parents. If she waits too much longer the rape kit will be no good. Without that, you don't have too much chance of proving it. It will be her word against his. A rape kit will PROVE he did it.