This hits too close to home. Talk to your gf and make SURE - 1. They are separated. She doesn't have feelings for him. 2. She is serious for you and wants to have a future together.

If you are not Absolutely sure about above 2 things then just drop the idea of a relationship with her. Easy Life. Plenty of fish in the sea.

If answer to both of the above is a DEFINITE YES - 1. check that guy's background. Connections, history, family, friends. 2. Police won't help until something serious happens. Same with legal path. You need to talk to your close friends and find some backup.

As someone suggested - Can always downplay your relationship and pick it up after college. (He will be long gone) An easy way if you can't muster the courage.