You have to talk with multipule characters before rescuing Cassie, first you talk to Arlene, second, if you completed fawns route (up to the part where she shows you she works at the club) arange a meeting with them, and then after that, go to the slums. go to the area with the guy who lets you sleep for 5 credits, an event tile should arise, after that you should figure it out on your own.