You can borrow a rig for him ahead of time, provide a copy of the Lefty or Wulff videos, or just point him in the direction of some websites that teach casting.
Email him a shopping list of flies and essentials to pick up at a local shop. It's key that he has his own trimmers, extra leaders, a spool of tippet, etc.
Before you step in the river make sure he knows how to tie on a fly and add tippet to a leader.
Practice is pretty fun; encourage that in the days prior to the trip. Then when you get there, show some basics about reading the stream, ethics about crowding other fishermen, then let him make his own mistakes in private.
Not to oversimplify, but what a great way to wade through tough times. Check in with him periodically on the stream. You don't have to fish elbow to elbow. The solitude is half the joy.
That's my 2 cents.