> people in the advocacy space use it to mean basic things like being well behaved or being nice to people

Thank you for this comment. As a family member of a young AuDHD person I am continually battered by a combination of their behavior (which directly and un-ignorably degrades the experience of everyone around) and by various 'advocacy' ideas that use phrasing like that to not-so-subtly imply that it's actually our bad for daring to expect to "not be hit," or to not be forced to constantly base our whole lives on one person's preferences, etc.

It's tough sometimes to figure out what's real, what's a fair expectation, what's something that can be learned/unlearned, and what's just inevitable.

Some kids are just really wild by temperament and it shows up in their first year or two on the Earth.

Short of breaking their spirit there's not much the parents can do to make them less wild. Parents have to try to find a way to let the kid be wild and feel good about being a temperamentally wild kid while also recognizing that there are limits and boundaries. A similar experience is perhaps owning a dog that needs to run around outside every day.

But nothing in the autism or ADHD diagnoses says you can't learn social skills or that they shouldn't. Nor does it require anyone to endure abuse or physical attacks.

Assuming you're not the parent (since you say the more generic "family member"), probably the best thing you can do is try to befriend this kid and learn a bit about what's like to be them. And try to find traits in yourself that help you see why they act one way or another.

Another thing to keep in mind is that things you take for granted may be very distressing to this person. They may be able to hear noises or smell smells or taste flavors that you can't. The parents have a tough job because nobody wants their kid's preferences to dictate everything, but nobody wants to knowingly make their kid distressed when they can help it. That becomes a tough tradeoff when you have someone who is easily distressed by a world that isn't really designed with their brains in mind. This is where things like earplugs and sunglasses can help. It's a bit like being hungover all the time. If you reduce the sensory input everyone becomes less irritable and more even keel.

Having had personal experience with this problem, there is actually a simple but difficult solution to your problem:

Focus in the effect if the person's action on you, and set hard boundaries for yourself.

Note that boundaries are in this specific form:

"If you do X in Y situation, I will do Z"

The consequence to the instigating person (the Z part) is important. The person you described likely has a deficit in empathy, so feedback on actions has to happen to modify behavior.

Note that I'm not saying to hit back. Consequences are generally best when withdrawing something of value than adding something of negative value.

For example: "If we're watching a movie and you are physically rowdy, you will have to leave the living room and entertain yourself in your room".

Focua on how things affect you, and what actions you can take in response to boundary breaking behavior. Then be ruthlessly firm and consistent about it.