The major difference in these communications is the level of intimacy and the range of subject matters that will be touched upon in communications. This is tied up in the fact that the goal of the employer-employee relationship is the benefit of the firm as well as in the fact that there is a power disparity in that relationship.
In a friendship, the two friends are on an equal footing. Friends do not need to worry about saying things that might hurt their careers. They do not have to constantly think about maintaining proper discipline and distance in their conversations the way a boss would have to when talking to a subordinate. Therefore, their communications will be more intimate and more trusting.
In addition, friendly communications have no tangible goals in mind. They are not as purposive. Therefore, they will range across a variety of subject matters while employer-employee communications will focus on the job.
Of course, this is somewhat overstated as there will be situations in which bosses and employees will talk about non-job related topics. However, the difference in their powers within the relationship will always have an impact on the way they communicate.