My friend is getting into a bad relationship. How do I tell her to avoid him?

I am currently a junior in high school and my friend, let's call her Wendy, is also a junior in high school. About a month ago, her step-sister, let's call her Emily, and her boyfriend wanted Wendy to meet one of the boyfriend's friends. Wendy agreed and the friend, let's call him William, met. Wendy and William took a liking for each other and after 2 days(yes, I said 2 days), he asked her out. I was, naturally happy for her, because that's what friends do. But recently this guy has seemed to not be great. In fact, he seems like a total ******. Emily heard from the grape vine that William was cheating on Wendy. Of course, she had to make sure it wasn't just rumors, but she seriously believed it. William completely denied it, and they went on with their lives. Then, Wendy broke up with William, yet she felt guilty for it. According to Wendy, there was too much drama and she didn't want to deal with it. I thought that she would be done with him. But they still remained friends. In the past 2 weeks, William has been nagging on Wendy about doing 'stuff.' Wendy had some traumatizing events happen to her before, and I don't think she wants to be reminded of any of those. Last night, Wendy slept over at William's house and apparently was forcing her into actually having sex. That worried me big time and now I'm scared that he's going to take advantage of her. I want to tell her to stop seeing this guy or at least warn her, but how do I tell her? What would you do in this position?