I'm all for people believing what they want to believe but please don't say just because you become a christian you've been "saved". You were fine before. I'm sick of hearing that from christian people. Just because I'm not christian does not mean I need saving!!
in reference to that other girl talking about your friends being saved.
Also I don't think there's anything you can do, if it's helping them be better people just let them believe however they like.
Assuming I'm reading this question right, do you mean like the mother in the Stephen King classic "Carrie"? It could be a form of obsessive compulsive disorder. If it wasn't religion, they'd go overboard obsessing about something else. Short of committing them to the mental ward of a hospital there's not much you can do. Like many other problems, they have to realize they even have a problem before they can even start to deal with it.
The religious call it being saved, finding enlightenment, or finding God. If you do not believe in a greater being, you will never understand. Sanity? What exactly has this person been doing? preaching in the streets?