What a terribly sad situation for your sister and her dog. :-(
This poor dog is absolutely miserable so it's time to see a specialist if that hasn't yet been done. A university with a strong vet program would be a good option. Forget about the breeder being of any help.
I wonder if the prednisone is at the core of the eating issue. It's extremely hard on the stomach, and dogs have a tough time with steroids. He might do better with a long-acting injectible form rather than an oral pill.
Since he has so many environmental allergies, it may be in his best interest to find re-home him in a different area of the country. If grass is big issue, he might do better in the desert Southwest. It may be worth it to contact Golden rescue organizations for help in finding him a new home. Your sister obviously has her dog's best interests at heart with all the medical treatments she invested in, so a last resort before electing to euthanize would be to find a home he can live in more comfortably. I imagine the word "heartache" doesn't begin to describe the situation. :-(
I know that my opinion on things like this is probably not going to be a popular one, but my theory on people who deliberately crossbreed dogs to get mixes with cutesy sounding names are doing a lot more harm to the dog world than they are good.
There are so many homeless dogs today, but people will still deliberately breed dogs like "Golden Doodles" and "Labradoodles" because people will buy them. The bottom line, however, is that these are not breeds, they are mixes, and the people who breed them purposely are no more responsible breeders than those people who allow their unspayued females to wander around outside when they're in season to be bred by the first friendly neighborhood stray that comes along, so it's no wonder they're not returning her calls. And because this is not an actual breed, there's not even a parent club you can report them, to because the Golden Retriever Club of America and the Poodle Club of America don't sanction cross breeding these dogs, so they are not going to stand by this "breeder".
I feel SO badly for your sister, because she is trying to do what is best for her dog, and naturally she loves him, and it's heartbreaking for someone to love a pet and not be able to do anything to help him. I wish I could shed some light on the situation for you, but right at the moment, I'm afraid I have nothing to offer.