Curiosity may propel you to know the applications that made any friends you have installed on his Android until you see it, and why not try it on your phone too, so it's not impossible with a nice trick depends on the knowledge of de ID for any private account calculates Google Plus friends who want to know used in phone applications.
First you will need to access Google Plus account for the person who want to learn the applications that use them, and then in to the account, click on the image to open his account, and you will see the ID's that person at the top are copying..
After copying the ID past it after the equal sign on the link below :
play.google.com/store/people/details?id=id
After that enter on the link and see the programs which install on him device like on the picture below :
If you like this idea please share it with your friend..