What would you do?

Cake • It's called the American dream because you have to be asleep to believe it.
-- I had three friends during elementary school, middle school and high school. We had other friends in high school too, but the four of us remained very close. For the sake of identities, we'll call them Lilly, Susan and Jade. 
-- Susan moved out of state and fell in love with a guy who I feel is a douche - for the basic reasons why anyone would think this about a person. 
-- We fell out of touch until last year - we started chatting on fb. I mentioned my studies in to The Craft and how excited I was to finally find a religion that suited me. Long story short, she flipped out and demanded I forgo my beliefs - or else she wouldn't be friends with me anymore. I told her no and that this religion is what I've before looking for my entire life. 
-- She blocked me on fb and we haven't spoken again since. 
-- About a month after that, I tried reconnecting with Jade, but to this day she's never answered any of my comments or fb messages. I don't know why. 
-- Lilly is still living in state (not too far from where I live, actually), and we are still friends. She still keeps in contact with Susan from time to time. So here's where I need advice: according to Lilly, Susan has fallen in to some sort of extremist Christian cult. Like the bad kind. I'm worried about her, but she doesn't want anything to do with me. I could probably get her address through Lilly and send Susan a letter expressing my concern for her, but I'm not sure that's the right move. I just don't want her drinking the cool-aid! She's a very gulable person - coupled with being stubborn as a mule. What would you do if you friend of 16+ years was in that situation?? 
-- For the record, I don't have anything against Christianity - but the way Lilly was talking about it, this really sounded bad - like Susan could get hurt. Lilly (who is a devout baptist) even admitted that it sounded like one of those crazy cults. This is why I'm worried.