There is no official way to get rss feed for a google plus user. But if you want to subscribe updates of a google plus user, There’s a new service called Google Plus Feed available that turns a user’s stream into an RSS feed.You can keep updated with all the posts using this RSS feature.It is easy to create an RSS feed for a Google Plus profile.
Steps to get the rss feed of google plus user.
1.get the profile ID from the profile link of any user you want to subscribe

2.add it at the end of the following URL: http://plusfeed.appspot.com.
( http://plusfeed.appspot.com/111907644848132470120 )

3. Add this link in to your RSS reader or Google reader.