How do I get iOS GameCenter Sandbox to appear on my father-in-law's phone?

I live on the East coast and my father-in-law is on the West coast. He's a coder himself, from way back, though he's never developed for OSX or iOS.

I want him to join my testing group for my iOS game.

I can use Crashlytics to get my game on his iPhone, but it's a head-to-head game that relies on the Game Center Sandbox mode. His phone isn't develoment enabled, so the Sandbox options don't appear for him in settings.

How do I get the Sandbox options to show up on his phone?