yes! I've had experience of this. Most of my cats are from this persons cats. What really got me was they "humanised" the cats in that they they wouldn't dream of "De-sexing" them (spay/ neutering). Finally, when they had nearly thirty kittens running wild around their house, I talked them into spaying the female cats they wished to keep. I got them intouch with my vet who did a favour on the bill for them. I made up posters for good home for they kittens, and actually had tok post them all over Dublin! I put my phone number on them and guess what? the kittens got homes! These people thought it was very cruel to neuter males, they clentched their thighs at the thought of it, but when the males started to spray the house, the smell got too much so, they eventually listened to me, and got them done too.
The only way , or the final point that I repeated, the one thing that I think shuck these people into action was- I kept on about the law stepping in if they didn't sort it, the local S.P.C A. have the power to prosecute, which includes going to court, a ban and a fine!

Regarding your inlaws, I doubt very much if the vet said what you were told. Unless he wasn't told the full story. If the vet knew the extent of the situation, he himself would involve the local pound, rather than let an animal suffer. The lady doesn't have a sense of responsibility, so I doubt if she cared to contact a vet in the first place.
Her local S.P.C.A. or local no kill shelter can be informed anonymously, you know?