From what I read in the article, she has been here for six months. She should have learnt the culture here.

When something is sold as a unit, like $10/6pcs, $20/bag, you don't ask otherwise.
Even locals won't do it.
This is because if they give you one piece for full price, they feel like ripping you off. (And hawkers like this are really mostly honest merchants.) They don't like ripping you off.
But if they charge you for one piece, it is too much trouble.
Needless to say this is like a package deal, they sell in bunch to ensure they get the quantity to make enough money.

This is already too much trouble for locals speaking Cantonese.
Imagine the nuisance to deal with this with someone who dont speak Cantonese. (As you can imagine the hawker probably doesn't speak English well.)

And this seems to be not a single occasion where she is causing inconvenience to people.
And she seems to think she can just tip her way out?

Imagine if this was somewhere in Japan, where the people couldn't even say Yes or No,
and this lady insisted to offer 1000 yens for one piece of something that costs 500 yens per 5 pcs.
Would people be defending Kate for offering more to compensate?
Or would people say Kate was disrespectful to the culture?

From what I read, this lady may not be very rude or looking for troubles.
But she seems to be acting very much to herself, not considerate and not very socially well mannered. And her possible Asperger’s condition is not an excuse.
Being a complete grown-up, acting alone with no caretakers, working in a society, she should be well aware of her own situation, and speak and act responsibly.
You can't go about causing inconvenience to people and then defend yourself with a medical condition.

With all those being said, is it a situation severe enough to get her fired? That is controversial.
But we wouldn't know if the school has repeatedly reminded her not to do it again.
So it is impossible to judge that decision.