I liked swampboys reply because he is right. We often don't get all of the info needed and make assumptions. What exactly was promised or not promised?
Assuming the OP's son plays HS baseball, I am going to assume the friends son does also. If this happened in travel ball before HS, I do believe that it is done but I get the frustration.
This happens in HS frequently, and I have no issue with it. My son has been a "ringer" and has had "ringers" on his teams. Example, when son was in his sophmore summer, when he went to USA tryouts, the manager brought in 2003's while my son was a 2004. The scouts came over to watch the 2003's, many of these players drafted very early with much talent, and because of them they picked son to go to final tryouts in Houston that year. He didnt make the team but that event, a free plane ride to Houston and much exposure at 14 going on 15 followed him all through HS.
Sorry if this doesnt sound right, but folks need to get over the little league mentality. I want my player to play for a coach that is fair, but I also want him to play with someone who knows how to attract attention to all of the players, whether it be a college coach, or a MLB scout.
JMO