Dog snapped at daughters friend.

The dog clearly felt uncomfortable in her presence. Maybe she had the scent of another dog or cat, maybe the dog didn't trust her. No one can say why the dog reacted negatively to her, but from the OP we know the 3 year old dog was never aggressive before.

Why did the 13 year old persist in approaching and trying to pet the dog? Why didn't the 13 year old realise the dog didn't want her to approach? Why didn't the OP tell her to back off? These are the more important questions and ones we can get can definitive answers to as we share a language.

Calling for every animal that reacts when cornered after displaying clear signs of discomfort to be euthanised is pathetic. They are not emotionless possessions, they are sentient and respond instinctively. Exactly like humans, despite how much you stress our importance over them.

ETA: maybe answer the questions you quoted rather than shooting off in tangents.

Will you take on board a post from a "Millennial" whose grandparents (that you lived with/beside growing up) trained aggressive dogs with a bite history, and now works professionally with dogs, and whilst not as a trainer, would work closely with trainers/behaviourists precisely for aggressive behaviour? As well as owning aggressive/reactive dogs, and having had to make that horrid decision to BE?

We don't know what happened in the OP. Only those in the house know exactly what lead to that situation. I've seen more scenarios where the dog is giving clear signals that the behaviour of the person, or other animals, is pushing their boundaries, those boundaries get ignored and the dog reaches threshold. So in this case, it's very likely the teen was acting inappropriately (such as looming over a small dog, moving too fast and startling the dog, being an excessively loud and scary person, etc.) and was told to back off. That back off was ignored. The dog escalated the warning to what would be considered Level 1 on the Dunbar Scale.

Generally, Level 1 and 2 bites are what I encounter most often. They're usually a "please, I'm telling you something and you're not listening", and are thankfully, very much trainable. Most vets would refuse euth for a single incident. I would recommend the OP bring on a behaviourist if they're not actually sure what the friend did, in order to make sure the dog isn't put into that situation again.

Level 3 and up bites need serious consideration. Some situations are accidental and the situation surrounding them needs to be accounted for (for example, I met a girl in A&E with a Level 4 bite but she stuck her arm in front of her dog that was defending itself from another dog attacking it. The dog never meant to touch her and so it wouldn't be considered something to be concerned about from a risk-to-people point of view). Multiple Level 3s are also considered Level 3.5. This dog didn't come close to these.

However, if this is a new behaviour, then that "something" they're trying to communicate is very often pain or discomfort. One of the first subtle signs of pain is a change in behaviour and/or a lessening tolerance for the environment they're in. Always worth a vet visit first, as any professional worth their salt will send you there anyway.

Whilst I agree that treating dogs like people causes issues, this isn't that case. The old school approach was more "oh, the dog growled at you? Quit pestering it and leave it be." If anything, now it's more of a shock to * some * when the dog acts like the dog it is and not a human.

In this case, locking the dog away achieves nothing more than a quick plaster over. The real fix comes with what I wrote above; acknowledging dog communication methods, getting checked by the vet in case of an underlying reason, and then working with a professional.

Jaysus.....

It has been proven that this does not work. By overly punishing their communication methods you are more likely to push the animal to escalating their behaviour. Dominance theory has been long debunked. Its not treating a dog like a human, in fact its quite the opposite. You are learning that dog body language and human body language is different and should be treated differently. Understanding that the dog is a sentient being that can be taught and trained in more effective positive reinforcement ways.

By teaching children and adults to respect a dogs boundaries you are not saying that the dogs comfort supersedes theirs. You are teaching them how to safely interact with animals and that their desires do not mean they can just do what they want, when they want. Do you think that all dogs should just accept handling and touching from all humans at any time or they are aggressive?