Call the cops and file a report. Your neighbor is only financially responsible if the tree trimming company won't pay for the damages, and if that's the case your neighbors home owners insurance will have to pay for it. If they don't own up to it, file a small claim and let the judge decide. It sounds like you aren't friends with the neighbors anyway, and if they're going to treat you like this, no reason not to let the system work for you. And if your other neighbor refuses to tell the police the truth about what he saw, tell him that your lawyer will have him record a deposition under oath, and if he doesn't tell the truth he'll be a lot more involved than he ever wanted to be. Perjury is a felony.