Asking her out would be fine; the problem, as I already explained, is how. However I do agree with you that her answer was over the top, a simple "No." would be the best.

Whole thing is no issue.

It was clearly an issue to the Anon, check the last paragraph.

If you are gonna randomly strike up conversations you will get cooked sometimes.

He wasn't just striking up a conversation.

Additionally (and that's neither side's fault), mob mentality is a plague. She was in a group of four people; people typically behave worse in groups than alone.