(I'm assuming you mean browser-side JavaScript)
Ask him why, despite his infinite knowledge of JavaScript, it is still a good idea to use existing frameworks such as jQuery, Mootools, Prototype, etc.
Answer:
Good coders code, great coders reuse. Thousands of man hours have been poured into these libraries to abstract DOM capabilities away from browser specific implementations. There's no reason to go through all of the different browser DOM headaches yourself just to reinvent the fixes.
