It covers the story up to the point of her not taking the drink. Perhaps she just wanted to see if she could. I agree that getting him to buy the drink may be more significant than actually consuming it. Or it could simply be a way of chastising someone she didn't believe should be talking to her. Or, perhaps, she simply forgot.

For practical purposes, she's not different whether she takes the drink or not. It's still a waste of money. If she didn't forget, it's likely she simply got a kick out of ordering some guy around. Not a situation I anticipated, but certainly deserving the same response as the selfish drinker.