Reaction for Peace

I really need some positive input on this type of a situation--when a dog “at large” enters private property & causes the property person undo stress by acting in a very vicious & threatening manner; and, a neighbor heard, saw the commotion & recognizes the dog. The neighbor did a good gesture by calling the dog’s owner, but didn’t follow-up with telling the property person. 

Naturally, the property resident called the authorities, and in the meantime, called around in the area to identify the dog’s owner. The neighbor that witness the action stated “yes & I called the owner of the dog, but I am not going to tell you who it is.”

What would your reactions be to this type of a neighbor?