Humane euthanasia is not the worst fate that can befall an animal. That said, I wouldn't lie to the current owner.
Actually we don't know that it's been a pet. The breeder could have put up the CL post, and if that's the case IMO he is shirking his responsiblity.Although we think we know what's best for the animal, it's not our call to make right now. It's someone else's pet and they(supposedly) have been caring for it for some time and are making an effort to find someone else to care for it. They are not looking for someone to kill it.
There's a big difference between culling an animal because it is in constant pain, sick with a debilitating disease, or has other quality of life issues that will never improve, versus taking one that is healthy and could live a long life just to put it down. PETA also didn't limit its activities to wild animals, they got thousands of dogs and cats from shelters and other rescues over the years just so the animals could be euthed and the bodies tossed in a dumpster.Not that it's not "the right thing to do" to euthanize any animal for quality of life issues... but really, we could extend that to some of the PETA folks who would happily "adopt" free pets from any of us and put them down because "wild animals cannot be happy in captivity".