Start asking around for help, talk to Fawn, then Lin Lin, then Arlene, then the hobos, then talk to the hobos again (when the other one doesnt listen), and then i believe you're good to go
Ok in case I'm doing something wrong I'll tell you more specifically.
Once you see that Cassie has been kidnapped, talk to Fawn. Go to your apartment (if you own one) and go to sleep, then go to talk to Fawn again. Get the men's schedule, then go to Lin Lin and do the same. Ask Lin Lin for help with taking down the guards (i'm not sure if there's anything else you can do but thats what I chose), then talk to Arlene, the police officer, and ask her for help (if it doesn't work then you need to talk to Fawn again, and I apologize for that). She'll tell you that trouble need to be caused in the slums, so you will need to go talk to the hobos. Talk to the hobos, pay them, then go to the bed right next to them and sleep. Then talk to the hobos again, then find the hobo who will scam you (he's in the strip club), then get your money back, then go back to the hobos and pay them AGAIN for their services. Then go next to them, sleep AGAIN, then go talk to Arlene. She'll tell you that the hobos have began causing chaos, and after that, I believe you can go save Cassie.
sorry if i misunderstood the question lol