Do you have to “pay it forward”?

It’s been two times now that I’ve gone through the drive-thru with my friend, when the car in front of her bought her order.
Both times she didn’t pay it forward. She says she has no obligation to buy their stuff.
I think it’s mean of her not to, but she argues that she doesn’t want to pay more for the car behind her than what she would have paid for hers.
Should she be paying it forward or is she right?
What do you think?