Did she know that you had already given the ticket away because they hadn't turned up?
If she knows that, I think it's fine. Otherwise, if you see her, say 'oh, sorry you ended up paying for the ticket in the rush - the thing is as the film had started i had assumed she wasn't coming and given the ticket i had bought away'.
But her behaviour in telling you to buy a slush was pretty extraordinary, really. AND she should have been a lot more spologetic.
The whole situation is a result of her disorganisation - don't worry.