Is there any way your parents, or some other adult, could help you handle this? It is not an easy situation for an adult, and really hard for someone who is 15. Do you have another neighbor who could just happen to drop by and see the mess and talk to your negligent neighbor?
If you can't get...
If your humane society doesn't deal with "livestock," they should be able to tell you who does. If you tell them about the cat, they will respond and maybe find the chickens suffering.
I'm new at chickens, so I can't help other than to say that herbal treatments are questionable if the chicken needs antibiotics. There are several threads on here that give details about types and doses of antibiotics. Do a search.
Also, cajunlizz is right - there are agencies out there to...