LD+
You talk to Fawn about it ( complete her route up until the torture rack ), Go to Arlene (Fish Lady) then the Panda girl in the slums for help in invading the Don's Lair
LD
I barely remember but basically you have to charm the Don enough to buy Cassie back.
Viewing post in Main Quest Line - Lust Doll - Cassie Ending (Part 1)
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.