Bail. She can't control his behavior but she COULD tell him not to ever contact her again which is what a lot of guys get when they shoot their shot. In your case she is telling you she "can't speak for the future".

You're both under 20. The chances of you finding your one and only at that age are infinitesimally small. Once you leave high school there is an entire world of people to learn about. This girl has already admitted that she sees the possibility of a future without you in it- which means you can too. Bail. Out.

She can figure out how to deal with the guy. Basically every woman I have ever heard of this happening to has decided the guy gets cut off completely. Your girlfriend decided that she is going to play "will they won't they" with both you and this guy, they can be each others problems while you live a better life.

DO NOT let her come back under the pretense that "the guy ended up being a total creep"... the warning signs were there and she not only did not cut him off she actually told YOU that she didn't know what tomorrow brings. She can go to the police, and her friends and family for help. It's not your concern.

Live your best life for you man.