She always tips on what she would have eaten

[Maybo]

I'm watching a TV show called "2 Broke Girls". Max and Caroline are waitresses, and they have a conversation while working in their restaurant.

Caroline: Max, can you watch my table while I run to the restroom.
Max: I already am. I've been watching that model girl pretends she's eating for 30 minutes.
Caroline: Oh, she's nice. She always tips on what she would have eaten.

What did Caroline say "would have eaten" instead of "would eat"?

 

[Tarheel]

Because she doesn't eat anything. However, it doesn't make sense. We don't tip based on the amount of food we have eaten but on the amount of the bill