Here is the latest I've heard from Tvtome.com:
Update on Sharona and Monk's New Assistant
Monk's new sidekick will be named Natalie, a single mother with a daughter rather than a son. Sharona is out of town visiting her sick mother (as in "Game Show"), leaving the door way open for her return later in the season (or next year). Cross your fingers.
So there you have it. There is a possibility she'll come back.