How about... a pc stepper operated hour glass. Use some sensor to determine when all the sand has fallen. You can do trials and test for accuracy of various glasses or environmental effects..
I've also always wanted to make a clock out of spheres like billard balls, where the digits 1-9 are distributed over the surface. An 2 axis driver similar to an inverted ball mouse assembly under each sphere rotates to display the correct time.
Ooh, that would be cool. No offence, but if he doesn't know what a CNC mill is, or the three dimensional axis, then i think this may be a bit over his/her head.