Since I don't know the exact 'Family Guy Diane Story', Diane could be anyone. She might be a new character introduced for a single - episode plot. Maybe she's an old acquaintance of one of the main characters like Lois or Peter.
In the 'Seduced by Madness: Diane Borchardt Story', Diane Borchardt is the central figure. But without more context, we can't really know much more about her specific identity, like her occupation or background.