This man sounds as if he has some mental/psychological problems. They should never ever leave their dog unsupervised in the yard, because if this guy comes along, who knows what he will do.
And frankly, IT IS THEIR BUSINESS if this guy is taking pictures of their property. I'd call the authorities on him the next time her does more than glance at that yard!!! Yes, he may be ill, but he is dangerous, too, and they need to look out for their safety and the safety of their dog.