My neighbor just told me his dog had one of my chickens in his yard. He got her to release it and put it back over the fence. Its a bit beat up but will be fine - I'm glad he told me and have clipped their wings.
We knew that if we free ranged our chickens this was a risk. We have never found them over that fence and thought that they avoided it due to the dog, guess not always. I'm glad he told me as I can do something about it - clip their wings and inspect the fence though I suspect they could get over it again if they really wanted to.
Having said that, I don't think I would say anything. In your case, an animal died and there is no happy ending. While you are completely "in the right", people are emotional creatures and are often irrational, especially around pets. Perhaps even more so around cats. If you say anything, you will then have to explain more, like why you didn't bring the body to them so they could see if it was theirs. . . and where it was when you found it. . . I'd much rather play dumb/ignorant and save the relationship then have to lie about any of that.
James