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 help, I'm pretty sure that if you talked to your 11-year-old friend and explained in advance what you want to do and why, he will understand.
Please keep posting.