The point of your story is important. Small amounts of money are unimportant to people with more money. The cost of dog food is unimportant when your net worth can vary tens of thousands of dollars day to day due to market variation. It is hard to understand what you have not experienced. I was poor so I still pinch pennies even though it is silly for me to worry about it.

I know it is deep in your past but consider what might have happened if instead of letting the friendship fade away, you charged her for the coffee you bought her, like you did with the breast pump rental. She probably would not have minded or noticed if you charged her for both her coffee and yours. You could have been totally above board and when you paid you could just tell her, I going to bill you for this.