I've had that happen before when one of our cars was in a shop. It took a good cutting or maybe claybar cleaning to get it off. I forget which. I've also had overspray of tar from someone doing roofing nearby when I was parked at work before but that washed off pretty easy the same night. So I'm leary now every time people are doing that sort of work when I'm parked near by.
Pay the neighbor to get it washed and see what it really looks like. Maybe it's dirt and he's being paranoid.