Feed+ is a sample app available from the Chrome Web Store. You can use this app to create custom RSS feeds for any public account on the Google+ network.
It lets you create RSS feeds for individual Google+ account users. You also have the option to create a RSS feed for search results for a keyword.
Once the feed is generated, you can add it to your Google Reader account or your own choice of RSS Reader app.
Based on the comments, it might not work for Google+ accounts which have switched to vanity urls currently available to a select number of users on this platform.