Do you cheat delivery people out of their tip, just because a delivery charge is included? Would you be a part of someone else doing so?

Today a coworker wanted to leave money with me for food she was having delivered. She said she wanted all her change back because she was not tipping since the place charged a delivery fee.
I told her she would have to come up front and pay the guy herself, as I would have no part of stiffing someone on their tip.
She wasn’t happy, and walked away muttering under her breath about me being f***ing retarded. And she used the whole word, in case you’re wondering.
How would you have reacted?