I am so sorry @Orc What an utterly dreadful thing to happen. You must be all over the place right now. I hope you get some resolution from this, and some closure.
I am honestly wondering where this was/is, as something similar happened to someone I know. 2 years ago now. (Their dog died at a boarding kennels, aged only 5, with no underlying conditions). It was a dogs and cats boarding kennel, not far from us, and we used to take our 2 cats there, and my BFF used to take her 2 dogs.
Each time we picked them up, they seemed a bit nervous and jumpy, and felt lighter/thinner. We only used them 3 times over 1 year or so, (3 days, 5 days, and 8 days.) After the third time I was not happy to leave them there again, and at the time they were the only one within striking distance. (7 miles away... The next closest one was 20 miles away.) So we started using a sitter.
My friend stopped taking her dogs too, as they were the same when she picked them up. She also used them 3 times. We put the weirdness/jumpiness down to them being in a strange place, and them losing weight being down to them not liking the food. But something felt 'off' so we stopped taking them, and got 'sitters' in our home thereafter.
Then about a year and a half to 2 years ago, 2 dogs and a cat died on their watch over a few weeks, and an investigation team (not sure what they're called sorry) came to do a random spot check after someone complained, and they shut them down, while they investigated. They started looking into them and their practices, and the kennels and cages. The animals all lost weight while they were with them, and their 'pens' were grubby and cold. It was November, and 4 degrees C in the day (minus 2 at night) and none of the cages/pens - all outside - had any kind of heating on. (Though the dog and cat owners were promised it for their pets.)
Also, they found a number of dogs in the same pen, quite cramped, and not from the same owner. So 3 dogs in a pen the size of a small bathroom (like 7 x 6 feet,) all from different owners. Yet the owners had paid for and been promised their dog would have its own pen/cage. They discovered the same thing happened with a few cats, (although not so many cats, as cats fight a lot more with other cats than dogs do with other dogs.)
I don't know if they re-opened. (This was in the West Midlands...) Where are you @Orc ?