As another poster mentioned, since it's on an unencrypted network you can sniff all the traffic going to / from your mac address. (sort of like google did when they drove around taking street view pictures) I'd park outside a few hours a get a huge capture file then go sifting through it later.. Other than traffic which is encrypted at the application layer (ie traffic such as https, ssh, vpn, or any other apps which encrypt data themselves prior to transmission like FaceTime), you'll be able to see everything in clear text .
Sounds like you've gotten in touch with a receptive detective and if you could pull some personal data from the capture file it would likely help your case.
You should be able to sniff data going to/from your MAC address without actually connecting to the unencrypted network... not sure if kismet does that but wireshark (in promiscuous mode) does. Still as others have mentioned maybe run this over with the detective to ensure you're not violating any laws.
This is violating many laws and could very likely backfire and put him in jail.