Get a paper ticket. Make a stand-alone reservation for the return segment in UA C class that will be held until day of departure. Call UA reservations and add your ticket number from the paper ticket to your new stand-alone UA reservation. Take your paper ticket to AA on the day of departure and ask for an endorsement stamp to UA because you need to travel with a colleague on the UA flight. Go to UA check-in, hand them your passport, and give them your flight details. Only when the UA agent asks for your ticket, hand your paper ticket to UA. If agent balks, say the matter was sorted with UA reservations in advance through the ticket number documented in the reservation. Excellent chance for success. If it doesn't work, check-in for your confirmed AA flight.