Okay its like this, for each friend you have they have their own little Safari. Each person has their specific type, so one of your friends might have Grass another might have Fire. The types are determined by their FCs so they will never change unless your friend gets a new 3DS. Each Safari has a set of 3 Pokemon in it. For instance one of my friends is Grass and the three Pokemon I can catch in her are Quilladen, Pansage, and Sawsbuck. These three Pokemon will never change. Also it seems what Pokemon are found is based on how far they are in the game when you first get to the Friends Safari.