OSX multi-user remote login w/ GUI
Yes, it is possible to give a user restricted access to your Mac. That's what parental controls are for (http://support.apple.com/kb/PH11471):


  You may not want all users to have unrestricted access to a computer,
  the apps on it, or content from the Internet. For example, you may
  want to prevent some users from changing their user settings or limit
  the apps they can use. To manage what a user can do on a computer, you
  use parental controls.


To configure it, as explained in the link above, create a new user for your colleague and turn on parental controls:


Choose Apple menu > System Preferences, click Users &amp; Groups, click the lock if it’s not unlocked, and enter an administrator name and password. Click Add (+) below the list of users, choose “Managed with Parental Controls”:



from the New Account pop-up menu, enter name and password information, and then click Create User. Click Open Parental Controls to set up the controls:

 

To restrict the apps the new user can open, click the App tab and limit the applications the new user can open. You also could select simple Finder to further restrict the new user's environment:




Now configure your Mac so that two people can use it at the same time:


Enable Screen Sharing in System Preferences>Sharing:


One of you must log in remotely using VNC. Install it on another computer and connect to your Mac. You will get best results if both computers are on the same network and if the apps you run don't need much graphic power.