This sounds like a big problem. I think you should talk with your girlfriend and see what her intensions are. I mean make sure she is with him only for this reason (which I doubt) and not any other reason like she has got some new feelings for her ex. A person shouldn't be with another person that threathens him  .It is wrong and it is not the right way to base a relationship.

If she feel like she is being held captive then you need to get another side involved some kind of proffesional hel since he surly needs help.