If the fiance is smart he'll dump this kook.
Most misery is self-induced. Wrong-thinking merely prolongs miserable consequences. Some people advance in that wrong thinking into a frantic search for happiness, ping-ponging between self pity to exuberance. Both are symptoms of arrogance allowed to advance without check resulting in misery to the person who has refused in their own volition to return to God by His protocol for life.
Her fiance, if he truly loves her, probably knows her better than any other person and his role might not be to separate, but to love her by appropriate response. If she continues to rebel there might be appropriate times to separate (i.e. dump her, but it probably isn;t appropriate to take an action which might condone her behavior linked to her bad decisions),...i.e. don't marry her after she just ping pongs back to self pity, remorse, and emotional lows in order to say things are back to normal,...that type of normalacy is self-induced misery.