I was going to write something along the lines of what MissE has written. Before having kids I was a vet nurse for many years, and there are a few different conditions that can cause what you might be seeing. Maybe you could just ask your neighbour if the cat is sick because you've noticed it's quite skinny. You could tell them you were just concerned that if it was sick you didn't want to feed it something that might make it worse. If they tell you there's nothing wrong with the cat, then you might want to consider calling the local council and seeing if they will come and collect the cat while it's in your yard. At least then if the owners are neglecting the cat they will have to stand up and take responsibility for it in order to get it back. Hopefully if it is sick then the council/local pound would make sure the owners got the cat the treatment it requires.