This may seem like a simple question, but have you checked the "QuickCorrect" settings on one of the offending computers?
To check these settings:
1. Open a new email message (like you're creating a new message).
2. Place your cursor in the Message area of the email.
3. Click Tools -> QuickCorrect from the menu bar.
4. Scroll down to see if "update" is in the left column. If it's there and "Friday" is in the right column, delete it.
Be sure to check for these settings on one of the computers which is having problems. Customized QuickCorrect settings are saved on each computer, not on the server. In other words, changing the QuickCorrect on one computer will not change it on another.
Good luck,
-Ron