The problem isn’t that she re-homed the dog. If you can no longer care for a dog, that’s the responsible thing to do.* The problem isn’t that she claims the dog developed behavioral issues - I have no doubt that can happen to a dog who was fine in the shelter, and fine for a while after they adopted him. I have no trouble believing that behavioral issues in dogs, like in humans, develop over time, and are often catalyzed by a small thing that makes the problem exponentially worse.
The problem is that everything she has said about it has been so histrionic and (very likely) so exaggerated. She obviously wanted sympathy for being in such a difficult position and for NO ONE to be able to judge her decision. But in the process, she implied that the shelter was irresponsible, that Lamby is beyond help (what do you put his chances of being adopted at now?), and then her dog trainer implied that all shelter dogs have behavioral issues that aren’t present in the shelter.
The problem isn’t that Lena re-homed a dog. It’s that she’s an asshole.
*I personally think it’s likely that she got the new dogs, Lamby had trouble adjusting, and she chose to keep the new dogs. But even with giving her the benefit of the doubt on this, she’s still an asshole.