Severely Underfed Cat? - How to bring up to HO?

A while back I was sitting for an overweight cat, but the owners were clearly aware and working with a vet. While I might have had some additional “advice” based on my own experience with a chubby cat, I kept it to myself.

However, in this case – not going to the vet – is a major red flag. So in your shoes, after the reviews were in, I probably would be a busybody and send a message: postal, email, text etc warning them about the various problems the cat will have at some point if they don’t do something. I’d probably use links to trusted sites, so it doesn’t seem I’m coming at this based on my opinion or even solely on my experience. If you don’t care about going back to that sit, you’ve got nothing to lose.