We all crave attention... to have someone to listen to us... be there for us etc... It's human nature. Some of us settle for temporary situations... or for the first person to come across seemingly willing to be this savior. It's not that she's become a slut. She's simply the ideal prey for most men. There'll always be a taker waiting to take advantage of someone in her position. She'll see them as the perfect fit to fill that gap. Once they've got what they wanted out of her, they move on, leaving your friend in the same position she was in to begin with... and once more... looking for the next guy. And this will repeat itself over and over again.
Not to belittle your friend in any ways, but I would suggest therapy. That solution won't be met without any resistance though. You can always try to reason with her to admit that the path she has chosen to walk is one that will most likely not have a happy ending. Few years ago, I would have suggested to let her learn from her mistakes... getting hurt and enduring the pain on her own will be lesson enough... but with so much more at stake, mainly STD's... as a good friend, you should definitely not let it happen any longer... even if it means talking to someone with more authority... her parents... even if it might be at the cost of your friendship.