When she fell in love with that person, she was never willing to spend a penny from him. Because of this, that person always called her stupid.

Other people's girlfriends expect to spend more money from their boyfriends, but she doesn't even want him to spend money for her.

From that person's point of view, it's only natural for his girlfriend to spend his money. Anyway, he didn't earn the money, so it doesn't hurt to spend it.