Someone Tied Messeges around my sisters cats Neck?

My sister has an indoor/outdoor cat and some idiot has tied messages around the cats neck threatening her to be a responsible cat owner and that he has killed quail 2 days in a row (he's a cat what do you expect!) and that the next time he catches her cat he will bring it to the pound. Now we don't know if this guy knows that my sister is the owner or if he doesn't know who the owner is. But her cat has his shots, is neutered and has a collar. What do you guys think about this? Is there anything she can do? She has no idea who this guy is. Should she even bother reporting these treats to the cops?