Speak with Lin Lin, Fawn, and Sammy to get better odds of saving Cassie. You will also need to speak with Arlene in the police station. You will learn what to do next from her, and once you have done what she suggests, you will need to speak with her again, after having talked with Fawn about Cassie and the Don, as well as having discovered and gotten involved in the BDSM club. I can get into more specifics if you want, but i'm leaving out details so I don't spoil too much for you, hope this helps :)