In the beginning of this episode we learn that this Drennan is an old teammate who trained with Rondu at the same academy as cadets for what is presumed to be centuries ago.
Much time has passed since they crossed paths, but when a disguised Drennan encounters the Orbots, he causes trouble for them. When he reveals his identity and his surface plan, Rondu just trusts this guy and welcomes him back!
Rob and co. doubt his sincerity from the get-go, and they try to get him to discern the true intentions of Drennan. But Rondu politely dismisses Rob's warning, instead trusting his own judgment, based on his recollection of Drennan as the habitual prankster from their academy days.
The whole crew suffers tremendously when Drennan reveals his nature and his true plans... and by then, it's too late.
Despite Rondu's fatal flaw for nostalgia, I liked the fact that Rondu made a serious mistake in trusting Drennan, because it showed me that Rondu is fallible and thus believable as a character. It was also great to see how the Galactic Patrol and crew handle the crisis apart from the conventional method of brawn and cleverness.
This episode taught me- and still reminds me of the unwritten moral that it's up to me to truly know who my real friends are; that friendships based on nostalgia-as a substitute for maintaining close contact- with them- can be a deadly Achilles heel that's easy to exploit in so many costly ways.