You can easily accomplish user-based brightness setting with the app [EasyOnTheEyes][1], which is free and gets 5 stars on the Mac App Store.

Here's what you'll need to do:

 1. [Download the app][1].
 2. Launch it on your account (the account that should have higher brightness) and choose `Off` on the menubar. This way, it won't dim when you are signed in to your account.
 3. Sign into your coworker's account, make sure EasyOnTheEyes is installed and running in the menubar on his account (you might need to install it again for him depending on your settings).
 4. In System Preferences, choose `Users and Groups` and then the `Login Items` section. You'll need to add EasyOnTheEyes as a login item for his account, so it can dim automatically when he logs in.
 5. Make sure his account is highlighted in the left pane (i.e. you're making the changes for his account, not your own). Then hit the <kbd>+</kbd> button below the Login Items table view, find EasyOnTheEyes in the Applications folder, and double-click it to add it to the Login Items list.
 6. Now, `EasyOnTheEyes` should show up in the `These items will open automatically when you log in` list. Note: You'll probably want to make the `Hide` checkbox checked, so your coworker isn't distracted by the bouncing app icon every time he logs in, but it's up to him.
 7. In the menubar, choose preset `1` and drag the slider to however your coworker will like the brightness.

**That's it.** Your coworker will have an automatically dimmed screen whenever he logs into his account, but you won't. 
  [1]: https://itunes.apple.com/us/app/easyontheeyes/id799584781?mt=12