Walk away, no matter what you may feel. It's a bad situation and you shouldn't want anything to do with it.

If you feel tempted to help her, you already have done what you can. Tell her if she really wants out of her situation, and is being blackmailed/threatened, open up to the school and be honest about the situation. The professor is the one abusing his position of power, regardless of how willing she was to go along with it in the first place, but it's not your place to do anything further, you've expressed how you feel. It clearly doesn't matter to her as much as giving her dignity away for an extra step up on the ladder.

Hypothetical situation, you make it work, resolve this issue..how comfortable would you be trusting her in the professional world? Imagine she sees an opportunity she wants and all she has to do is sleep with the right person to get it. It's not as if it'll be out of character.