Need a Riot.
Go to the Beggar spot under the Dons home, this is only after Arlene, asks you specifically for a reason to go to the Slums, then Fawn, I'm not sure if you need her or not but I always get her help anyways.
Asks for 2k Credits, ignore that, or give it for an extra step but just say No till everyone agrees to help, it should only cost you 500 Credits.
Talk to Arlene, afterwards.
Or... Just... *nyeh*
https://lustdollplus.miraheze.org/wiki/Cassie%27s_rescue
There you go.