I'm a little upset that this girl would even mention to him I contacted her on Facebook
Um...remember the whole cult thing and like bazillions of threads on this site?
I think the conundrum you are experiencing is between trying to be friends with these folks and trying to not be part of the cult. I'm not sure it works that way.
But there's plenty of polite ways to say, "I'm sorry, but I don't believe in this religion any more. Wanna be friends still?"