Everyone makes mistakes, but, as a mechanic working for a living, that is a very bad mistake. Life endangering in fact. He sounds like he wants to make things right. I would let him pay for parts and have the job done by someone qualified. Removing brake rotors is a PIA due to locktite. Don't need someone buggering up the screws, or breaking one off, or rounding out the inside. Degree of difficulty and cost goes up a bunch if that happens. Personally, I would never let him touch anything of mine again. I do this stuff myself because, I work slowly, methodically, and pay attention to what I am doing. Many people could do it faster, but, when I'm done, I know it's done right. No need to be nasty to him. Hopefully, he will learn from his experience.