You could maybe ask sarah why her friend wants to leave. ask alot of questions like "why does your friend think it's a cult?" or "how would you know something is a cult?"
Here's a few more:
"are all cults bad?"
'what makes a cult a bad thing?'
"what are the characteristics of a cult? Do all cults only have one leader? Can a cult have more than one leader?"
"Do you think your friend might have mistaken some rules within the organization for the types of rules one might find in a destructive cult? If so, what rules?"
"If you were in a cult, how would you know?"
"what does your friend have problems with? What is she concerned about and why?"
Perhaps you can get Sarah talking about the friends concern and try to get her to free up her own thinking a bit. If you can get her to elaborate on the friends concerns, you might be able to loosen the mind-grip a bit and sense what concerns Sarah herself might have. You might want to avoid too much anti-org. stuff at first and try to connect with thinking person before dropping a few small seeds of doubt. Good luck!