Snail
I assume you have a very excellent legal background and understanding of the law.
The airport property owners, because of the new law, are exempt from liability due to accidents. The man that is no longer an airport property owner is not exempt from liability.
That is ok to protect you as the airport owner from other users who you have premised as public use, or specific permission use depending on the status of your strip public/private. This is very similar to most recreational use clauses. It takes the known risk of such activity and brings it back to the participant.
Your neighbor who is not part of the airport community would not be involved in that liability as he is not an owner of the airstrip. He has given nobody permission to use his property. Once you enter his land you are trespassing.
Worst case scenario. A family takes off toward the trees. Maybe it is very hot, maybe he has some sort of engine problem that slows his acceleration. In either case he does a high speed abort or lifts off but cannot outclimb the trees. He hits them at high speed and burns. The plane is on this man's property.
The survivors, though badly burned will be approached by attorneys. They will point out to the jury that this man deliberately planted these trees to cause a hazard for the airport. He was a pilot and knew the probable consequences. After seeing the pictures of the burned children, etc. do you not think the jury would find for them?
Whether or not the man deliberately planted the trees doesn't matter ....it's his property and he has the ability to do so. These trees should be marked in the AFD as a hazard of use of the runway. Should a pilot have an accident by taking off too short a strip, too heavy, or having an engine problem and hitting these trees, your neighbor would not be liable. In fact the pilot of the aircraft could be liable for damage to his land and any cleanup costs that are involved.
Should the FAA have requested that he cut down the trees as a matter of safety perhaps you would then have a case of negligence.
So yes somebody might be able to sue your neighbor (Heck this is 'Merica anyone can sue anyone) I think the countersuit for damage would have a lot better chance....