I am sure the local birds, racoons, bunnies etc will also appreciate the shower. NOT! The neighbor needs to get a life!
I have a cat ... I love cats. The neighbor has every right not to expect to find kitty poo if she digs in her own yard. On top of that, kitty urine can be pretty offensive. I have a feral tom that lives on the back of my property, and it is not nice where he hangs out, but I'm willing to share because he eats rats.