I think the only viable option is for that person to return the fish ASAP and not make them suffer anymore.
Just a difference of opinion then, I don't think that's the only option. Getting the water quality in that 3.5g okay so the fish survive is priority number one for me, then working on persuading them that a larger tank is what the fish need, and will be easier to maintain, or to rehome/return the fish once the tank is stable, and just keep shrimp or snails instead. But none of that can happen if the fish have died of ammonia poisoning within a day.
I try to be gentle to people who are new to the hobby (the fish keeping hobby, whether you call fishkeeping a hobby is semantics really, but it's a useful catch all term) because many have been misguided by fish stores, including in that thread. For people new to fishkeeping, it makes sense to ask a fish store. They
should know how to care for fish, it makes perfect sense to follow their advice. Then they end up with a tiny overstocked, uncycled tank, dying fish, and a sharp lesson in the fact that it's more complicated than adding fish to a box.
Berating people for following bad advice doesn't help them or the fish. The vast majority of people do not want to be cruel, aren't trying to be heartless, and will respond better to kindness and guidance rather than venom. Ideally, everyone would research any pet long before getting it, but I also don't entirely blame them for listening to a fish store and trusting bad advice, or googling and finding misinformation, there's certainly plenty out there.
I also believe that there isn't a fishkeeper out there who hasn't made a mistake. I know I've made my share, including ones that have killed fish. I feel terrible about that, but mistakes do happen... whether people admit them or not.
Telling them to just return the fish and give up isn't likely to work, they're more likely to ignore you, and the fish suffer as a result. Taking into account human nature and trying to work with less than ideal circumstances, while working towards better ones, is an approach that seems far more humane to me, for the person and for the fish.
@AilyNC can testify to this. She trusted a fish stores advice, and had an uncycled tank and was losing fish, and was sold fish totally unsuitable for the tank she had. She certainly wasn't heartless or cruel, she was just given bad advice. If she'd been attacked and berated and told to just return them, I don't know what that would have done to her, but it certainly wouldn't have helped anyone, especially the fish. She was sold huge plecos that need hundreds of gallons of water, and she took the time and effort, despite being very attached to those fish already, to find an aquarium that would take them, a long drive away. Had she returned those plecos, I can almost guarantee they wouldn't have nearly such a good home. She worked her butt off with daily water changes and water testing to save her other fish, learned a ton, now has two beautiful tanks, and is a valuable member of the forum, all in the space of a few short months. She's a credit to the hobby, and has valuable experience to share with other people who were in her shoes.
Not to take credit or anything, but I was one of the people on her first thread, helping her, and the fact she cared and as willing to do whatever it took was clear as day. Had she been greeting with hostility and deridement, I doubt she'd want to be here and learn anymore.