If I was going to butt into this situation, I would go to the mentor and mention that he should not be guiding your friend any longer, since what he's after would not be helping his marriage. I would mention to this man that he would put her life at risk, since no good can come of him having sex with her.
I would talk once more with your friend and advise that nothing good can come of this and tell her that you're worried about her. And then you step back and be a friend when this situation crumbles around her feet.
You cannot make either one listen to you and you risk losing her friendship if you were to butt in. Do what your heart tells you to do and then let it go.