She's lucky they bury it. My previous neighbours had cats, and it must have been them who'd crap on my lawn and leave it, as it stopped when they moved.
I could suggest a 12 bore, but other than that it's difficult to suggest much. What works on one cat may not on the next. I did have a mains powered cat scarer out the back which had some success. But a multitude of different things at once is probably the best bet. Try citrus peel, and maybe try that lion's crap pellets you can get. Try the mains operated scarers that emit sounds inaudible to the human ear. I'm told mothballs also deter. I know you can get systems that squirt water when movement is detected, but that is difficult out the front. Or hide with a large flat shovel in your hand waiting for the culprit.