Perhaps you could get your son alone and talk to him? You might let him know that if she continues with whatever may be wrong with her, when she is in school the teachers will notice. If he refuses to get her help then, they will have to call CPS, its the law.
I don't think you should threaten to call social services yet. But it is something to think of in the future. Are you prepared to take custody at some point? The fiance sounds like a problem.
Good luck! It is frustrating to see someone doing the wrong thing & you can't so much.