Show her this and explain to her bringing used parts to the mechanic means he has to clean and inspect everything. Brake line fittings are incredibly fragile for what they are and you have to be super careful when you unscrew them. Tell her you're sorry that you didn't do a better job and at least the mechanic didn't skimp on taking care of one of the most important parts of the car.
I find that when mine gets irritated over something irrational I just Fosters her off with something that is.