I don't think that Adrian did it out of a sense of rightness. He did it because he had no choice. Adrian was basically suspicious that he was being monitored by the FBI (he had just come out of a judgement where he was prohibited from using computers, so this is not an unreasonable thing to believe), so it was either say nothing and get two people in trouble or say something and keep it to just one.

Simply put, as much as I respect manning's actions, manning should not have confided in lamo. Rookie move.

source: I... know people that know Adrian