Julie,

Willows are notoriously not good at sealing off (aka "healing") rotting areas. This rot allows for the center of the wood to break down and eventually be hollow. Often the tree appears fine since the living part of it is more on the outside---just below the bark. Willow wood is also considered quite weak compared to other trees like an oak. I am not surprised parts of the tree are hollow, and storms did a number on it.

Filling tree cavities can either harm or do nothing to the tree---it rarely helps. It all depends on what it was filled with. In the old days folks would fill holes with concrete, which was really bad. Concrete can allow wet spots to never dry out fully, which can then cause more issues with rot. To make matters worse, when people cut the tree down, their chainsaws would be destroyed by the concrete.

I am not sure what filling with golf balls would do, but I suspect it would not be as bad as something like concrete, because there would be plenty of gaps for air. But again, what your neighbor allegedly did is very strange, I have never heard of such a thing.

To see how your willows are doing, I would recommend contacting a certified arborist. They have tools that can show how much rot or decay there is inside your willows. They can also help prune damaged or diseased branches and offer advice regarding its health. UMN Extension has a good resource on what to expect when hiring a tree professional.

I am not a lawyer, but this resource regarding "tree law" might be helpful for you as well---sometimes neighbors can be difficult to work with regarding trees.

Best,

Shane