I don't have teenagers but I certainly read D H Laurence, Harold Robbins, Jackie Collins, Shirley Conran, Mills & Boon, Marquis de Sade, hair raising autobiographies by Hollywood stars, nasty horror stories and some dodgy porn involving an Alsatian and a dwarf from my local library when I was a teenager.
My DM trusted me and the librarian. I was considered sensible and it was just part of my extensive reading habits (20 books a week) I soon learnt to be discerning, shared my parent's values and have a degree in English Literature. I still enjoy reading Jilly Cooper novels, alongside the latest Lionel Shriver or Hilary Mantel.
50 shades is exploitative tripe, but as long as it is balanced with other books, it will be nothing more than a badly written, easily forgotten laugh with her mates. There are plenty of erotic classics by well respected authors. Anais Nin anyone?
As long as your daughter shares your values, understands about the dangers of misogyny and exploitation, has self respect and intelligence, it is not going to harm her. She'll probably read enough of Fifty Shades of Shite to talk about it with her friends and discard it ASAP.
If you are really concerned that she is easily lead and succumbing to peer pressure the book group discussion at your own home with parental permission will frighten her more, especially if you suggest they all write a 500 word essay on relationship power politics, which they read aloud in your presence 