I had one of those issues a few weeks ago on DL. My girlfriend and I (who are long distance) were flying VPS-ATL together (she and I were flying to different work sites), and I got an upgrade (but she didn't).

It was an easy decision - switch with the person who was sitting next to her (in a middle seat) and request permission from the FA. We each got a bottle of water, some time to chat, and probably made some infrequent traveler happy.