I just recently went through a similar experience about 2 1/2 weeks ago. Mine involved a dog though. My husband and I agreed to dog sit for an old friend of his. As soon as the dog walked in the front door my jaw dropped. The poor thing was skin and bones. I didn't say anything right away because I wanted to think about the best way of going about it.
Anyways, my suggestion would be to do what I did and if that doesn't work then you can decide if you need to take further action. I had my husband contact him while he was on vacation and give suggestions in regards to feeding more, taking the dog to the vet, etc. (I figured this way he couldn't get too upset since he was in another state and he couldn't run right over and take the dog back). Throughout the conversation my husband never blamed him but rather suggested that maybe he didn't have the time for a dog right now. This guy can get offended very easy so it was a slow process. In the end he agreed and now we have 3 dogs. (2 rescues within the past 3 months) My point is, sometimes pointing a finger and blaming does not work as well as suggesting that maybe the time isn't right for a pet. They will figure out once they let the pet go that it was the right thing to do, and you will also learn that you were right in your decision. He has yet to call and ask about the dog. She is gaining weight and is happy so that is all that matters in the end.
Anyways, my suggestion would be to do what I did and if that doesn't work then you can decide if you need to take further action. I had my husband contact him while he was on vacation and give suggestions in regards to feeding more, taking the dog to the vet, etc. (I figured this way he couldn't get too upset since he was in another state and he couldn't run right over and take the dog back). Throughout the conversation my husband never blamed him but rather suggested that maybe he didn't have the time for a dog right now. This guy can get offended very easy so it was a slow process. In the end he agreed and now we have 3 dogs. (2 rescues within the past 3 months) My point is, sometimes pointing a finger and blaming does not work as well as suggesting that maybe the time isn't right for a pet. They will figure out once they let the pet go that it was the right thing to do, and you will also learn that you were right in your decision. He has yet to call and ask about the dog. She is gaining weight and is happy so that is all that matters in the end.