Oh no, so sad to hear.
To give the benefit of the doubt: maybe she was a stray who just showed up at their place one day?? Also, in many countries and cultures, it's not the norm to treat cats and dogs like family members the way a lot of us do.
That said, it's probably not the norm to have a cat or dog and just not feed it...
I agree with the others lausa. I would absolutely not worry about just taking the cat in or trying to get her adopted. There's no way her owners would know what happened, and given the fact that they don't seem to be taking care of her, it's hard to imagine they'd be very upset.
If it turns out that this is all a big misunderstanding, and they put flyers up or something, you could always say that she showed up skinny, and because she didn't have a collar, you assumed she was a stray.
I've got a stray in a separate room upstairs - she showed up 4 weeks ago absolutely emaciated. And front and back declawed.
I posted on craiglist that if someone had lost the kitty, I'd try to catch her (even though she was in my house) so that I could try to weed out anyone who gave me bad vibes.
Your skinny kitty is lucky to have found someone who cares more about her than her owners!