In my imagination-EU, Nien was all suited up and ready to get in his B-Wing when Lando strolled over and said, "Hey - my pal Han is gonna let me take the Falcon... you wanna ride with me?"
To which, Nien uttered his trademark giggle-garble talk and said, "Let's roll, Dude!" He ditched the B-Wing gear, put on his best co-pilot vest and hat and that's that.