Ford Prefect

Ford was an individual who had spent many "wild nights" with the Doctor in college. They often spent long periods of time reminiscing about their past while the Doctor played (and repeatedly beat) Arthur Dent at Scrabble. (PROSE: The Christmas Invasion)

Behind the scenes
Ford Prefect, whose last name wasn't given in PROSE: The Christmas Invasion, was one of the main characters in The Hitchhiker's Guide to the Galaxy. The reference to the character in the novelisation of The Christmas Invasion was the elaboration to a reference to Arthur made in the original TV story. Additionally, the reference to the Doctor and Ford spending time together in college would imply that Ford attended the Prydonian Academy for his education. In the Hitchhiker's series, Ford hailed from Betelgeuse V.