In my town, a house fire started from a bad wiring splice a homeowner did. This is part of a pattern where folks skip safety to save a few bucks. You should always explain why it's dangerous. That quick talk can change how they handle things later. Just billing them fixes it now, but teaching them stops it for good. Taking that moment to care does more than any invoice.
Wait, they were trying to save money by doing their own wiring? That's insane. You can't put a price on not burning your house down. A quick chat is the absolute least you can do for someone that reckless.