First of all, she is the one who has to make a decision and take action if she chooses to. It sounds like she is making a choice to let this guy use her and take advantage of her, and maybe she has assumed that since she let him before, it is too late to stop it. It really doesn't sound like she wants to be having sex with him. Maybe the next time she talks about it, you could ask her that: "Do you want to keep having sex with this guy?" If she says yes, you could ask her to stop complaining to you, since she has made her choice. If she says no, ask her why she is doing it. Don't tell her what to do, but you can ask her questions to help her to work it out, reflect what she is saying back to her, ask her what she would tell you if you told her the same thing was happening to you, etc.