Some cats are much larger in their own minds than they are in reality - cats like this will often stalk prey that is much bigger, at least until that "prey" turns on them. We had a cat for a long time that stalked and chased deer - because the deer ran away from him. One of my barn cats is most certain that he would like to attack a chicken but he is all hot air - he climbs into the chickens' free-range area and stalks the chickens as long as I am looking....but if a chicken looks at him, he moseys off like he has no interest. He moves much more quickly if the chicken comes directly at him. So much depends on the cat and on your chickens - in my experience, if the chickens don't run or if they fight back, the cat will only try catching them once. Since it is not your cat, you have to err on the side of caution and cage your chickens or convince your neighbors that they need to keep their cat on their property. Good luck!