Good points.
Personally, I have never been in this situation of having to foot the bill without having consented to it beforehand. Contrary to what I have often read on this forum, freeloading is not, in my experience, something Thais will typically do and they will more often have a tendency to be "greng jai", i.e. feel embarrassed/awkward in the situation the OP described.
But this woman's attitude definitely was out of order and I understand where the OP is coming from (though I have long stopped being confrontational in Thailand myself – In my opinion, there are more effective ways to deal with such situations privately, with no loss of face involved for anyone).