The cat was necessary to control bugs, snakes and rodents. 

Ants, termites, spiders, wasps/yellow jackets/hornets, slugs, snakes, moles, raccoons, etc.... know what is common in your area and learn best practices for dealing with them.These are basics... of course.

If it was a newer system , I would dig down to a drainfield pipe and cut out a section and run a sewer snake down the drainfield , then I would follow with caustic soda down the drainfield .

They would simply have a non-working toilet and have to call a plumber to snake it. 

Chalk, screws, nails, putty, spare knobs for the shower and sinks, plunger, snake and wet-dry vacuum.  

@Haydn Zeis brought the thunder. 

Snaking a clogged drain is cheap. 

It is not "snake oil."

Contact whatever licensing board is appropriate and sue them in small claims court.Had a similar situation at my daughter's rental property, where the PMC did the video scope that found cracked sewer line blah blah.We hired someone to snake the pipes - problem solved for about $100 as opposed to the $5,000 they were quoting including tearing up the street and city permits on and on.

Now, if she was stuffing "flushable" wipes, and we all know there really is no such thing, and you've explained to her that she can't flush flushable wipes and if it happens again and the snake brings up a wad of flushable wipes again - then she'll have to pay the bill - that's reasonable.But, to charge her for a problem because you can't figure out how it's happening - well, you'll end up with an expensive consequence.