Hmm, whenever I've seen a dog kill a chicken, there is blood, not just feathers. Also, they haven't usually eaten the whole chicken, in my experience they are most likely to leave the body behind after playing with it. Hopefully your chicken is just totally spooked and hiding or wandering around somewhere -- whenever mine get a big surprise or fright they drop a whole bunch of feathers (it's a distraction/defensive technique.) Pits aren't really bird or hunting dogs, so unless it was specifically trained to go after little animals in a previous "home" then it is not really in its makeup to just go after a chicken... Not like my mom's big poodles, who will do anything they can to get one of my chickens! And even they aren't usually fast enough. So keep some hope.
As far as the fence goes, if their dog jumps the fence repeatedly, they probably need a stronger collar, like a training collar, or the collar that the dog has on is not on tight enough (ie: its not making contact.) The owners may not realize an adjustement can or should be made. It's a learning process -- we've had invisble fencing with an ever changing pack of up to 10 dogs at a time over the last 15 years, and every dog is different. Few dogs will repeatedly jump the fence if the collar is really working right. That type of fencing is a big investment, and continues to cost money forever, so the owners clearly are making a real effort at keeping the dog in, and I'm sure would be receptive to any reasonable concerns or suggestions.
Good luck.