Agree with the last post. It's just really awkward and uncomfortable for everyone else. Like someone who meets a new acquaintance and starts sharing *way* too much information. People feel obligated to respond in some way, and often the person posting gets very upset when they don't get as much support/sympathy as they want.
I would suggest the friend's sister start a blog where she can write this stuff down, and maybe post the link to it on an abuse survivor's group--somewhere with people who don't know her personally, and/or have more experience with this kind of thing.
Yes, I know that what the survivor went through is much worse than the awkwardness of reading it, and I'm not saying people who've been abused should be quiet about it. But I just don't think posting it on Facebook in detail is a great idea, for lots of reasons.