Night Rider's Lament has a great yodeling part but the song is a little on the hard side to learn.It's not your typical cowboy song, you might try and go with Ride an old Paint, or a more known and add some nice yodeling. HAve you asked the nice gentleman if he has his favorites to yodel to. He can probably answer the call since he's the yodeler.
Good luck with the gig, sounds like a good one.
Mudjack