In the tie-in novel Dale Cooper My Life And Tapes, Diane is identified as Dale Cooper's PA and secretary
Best explanation is that Diane is Cooper's secretary at the FBI. Every time he fills a whole tape he mails it to her and she transcribes it, takes notes, etc. We see him greet her in the office in the movie (FWWM) but even then we don't actually see her. It should also be taken into consideration that when Cooper asks Diane for earplugs, he receives them almost instantly.