Scholar: how to edit main-nav.php in child theme?

Hi:

I hired a designer, who's not familiar with WPMU themes, but who knows more php and css than I do, to make some custom changes to the theme that I couldn't manage.

site is: http://few.modernmediajapan.com/

I needed some navigation bar changes, which he did, but his question is:

how can main-navigation.php file be edited in child theme and take navigation from there not from parent theme?

of course, everything should be able to be edited in the Child theme, but I'm not sure what to tell him so that he can do it that way.

looking forward to your reply,

thanks,
Terri