You can also buy cat repellants at garden stores. It's a grainy type stuff, and cat's hate the smell of it. When we had our neighbours cats in our gardens, our vet told us to check it out. It worked, but the only thing was eventually the cats would just bear with the smell and go in there anyway. She could also try tin foil all around the patio.. maybe if the cats can't get under there they will stop coming (cats don't like to walk on tin foil).