Tristan and Buford met when the latter was walking through the forest at night with his lucky tambourine. He quickly gained the whalemingo's trust with a pack of shrimp jerky that he had in his pocket, and the two bonded. Eventually, though, Tristan had to return to his home planet.
In the episode, the other children, Baljeet especially, doubt Buford's story of his alien encounter until the Whalemingo planet, drawn in by Doofenshmirtz's invention, begins hurtling towards Earth. Buford immediately recognizes Tristan out of the thousands of other Whalemingos and at the end of the episode, gifts him his lucky tambourine as a token of their friendship.