Fortunately I wasn't involved, but I did see a couple being a problem on a Delta flight recently. The person with the real seat had to get the FA. When she asked to see their boarding passes, they said they gave them to the gate attendent. She then reminded them that they got the stubs back. They then made a big production out of trying to find the passes. I lost track of what was going on with them at this point, but I think they did end up having to move.