You're not going to want to hear this, but you should have left his passport with him. It is SOP for UA to collect passport information at check in for an international itinerary. No passport, no boarding. Unfortunately, that's where you went wrong here.

While I'm sure it's really frustrating, unfortunately I think this one indeed will fall under "the expensive lesson" and I doubt that you'll get compensation since this was all ultimately the result of your actions.