You can check-in online for any passenger as long as you know his/her record locator and last name without logging in online with his/her password.

Just learn this from web support.

United.com => Itineraries & check-in => check in online => Guest access, type in Confirmation or e-ticket number and then last name.

This is a great tool to double check your sponsoree’s itinerary in case there is a problem.