When this problem came up "Yet again" on uk.rec.gardening
One very good idea was to put in a small sand pit/box about 3 or 4 ft square
for the cats to use, they will go to it rather than the soil as long as it
is kept clean.
You could suggest this to your neighbour, otherwise put on in in your garden
where the cats will find it easily, whilst you will still have mess to
clear, it will be in one place and they wont be damaging the rest of the