Dianne Brooks (née Sooner) was a old high school friend of Trudy, and through her she knew Adrian Monk. She appears in the Season 5 episode "Mr. Monk and the Class Reunion".
During senior year, her mother died, which made Dianne depressed resulting in an attempt to commit suicide... She wrote a note and took pills. The school nurse Katherine Rutherford became concern for her and arrived in time to save Dianne. Dianne lived on, but kept the suicide note.
When Trudy was murdered, she attended her funeral. Some time after she married Kyle Brooks, a salesman at her father's company. Kyle was having an affair, and wanted to kill Dianne for her money. When they moved, he found her old suicide note and decided to use it as the perfect cover to kill Dianne. But since the note was 25 years old, there were thing mentioned in it, that wouldn't fit the circumstances, if he were to use it in a recent murder... During Dianne's high school reunion, he created situations, that would match the note, so that the authority wouldn't doubt the age of the note. Luckily Monk figured out his plan and managed to stop Kyle just before Dianne got killed... It's unknown what happed to her afterward, but it's fair to assume that she divorced her husband.