Upon their rescue, Arthur is astonished to find that there is another Human from Earth living on the spaceship Heart of Gold: Tricia McMillian, a woman he had tried and failed to date. It turns out that Zaphod, visiting Earth in disguise, had been the person who took her away, being much more interesting than the bland Arthur. Depending on the version (radio play, book, movie) they have had more or less interaction before meeting on the spaceship:
"I wasn't doing very well with her. I'd been trying all evening. Hell, she was something though. Beautiful, charming, devastatingly intelligent, at last I'd got her to myself for a bit and was plying her with a bit of talk when this friend of yours barges up and says 'Hey doll, is this guy boring you? Why don't you talk to me instead? I'm from a different planet.' I never saw her again."
(Adams, The Hitchhiker's Guide to the Galaxy, Google Books)
The odds against finding another human, much less one who Arthur has met in the past, are astronomical; the Heart of Gold, which runs on improbability, brings them together because of these odds, not despite them. As the book continues, Arthur continues to try and ingratiate himself with Trillian, regardless of his being unsuited to life in outer space.