You kinda want them to manage to get past the TSA so they can get arrested trying to illegally enter another country and have to decide if getting help from the state department is worth admitting they are a US citizen
Some countries, like Australia, require you to show the airline a valid visa before you fly. If you land without one, the airline is responsible for the cost to repatriate you. Obviously to get a visa, you'd need a valid passport.
I don't know about TSA (I've never not used my passport as ID when traveling internationally).
But the airlines will validate your documents before giving you a boarding pass. You have to either scan it into the app or show it to an agent at check-in.