This is a very confusing post but as best I can tell, two people were on the same itinerary and thus you'd think they'd receive consecutive boarding positions when they check in. For whatever reason, one person received A4 and one person B5. B5 tried to board with A4. Either these people thought it was ok for two people on the same itinerary to board with the better boarding position (which is simply not the case), or they willingly tried to cheat the system. Either way, they were caught and one passenger was told to board with B5. This should have been the end of the story. Instead, they apparently made a stink (including asking for the gate agent's name, and beginning to film the encounter via cell phone) and the result is this post.
If what I've typed above is correct, then I 100% agree that the belligerent passenger(s) should have not been permitted to board. Depending on the severity of the outburst, the person(s) possibly should have been preventing from flying. Without having seen the outburst or the gate agent's reaction to the whole situation, it is hard to judge the severity of the situation.