I believe there is an airside connection as well, provided by the airline, if you have an international to international connection. However it sounds to me like you both are getting off the respective (air)bus at Narita, in which case you both go through customs and immigration and then one of you uses the shuttle bus service referred to on the airport website to meet the other.

There are more efficient methods if you are both proceeding into Tokyo -- e.g., meet up on the Narita Express -- but these are difficult to coordinate without both having a Japanese cell phone (in case one flight or the other is late).