He should try to check whether his flight was oversold. Could it be that the mistake was an attempt to avoid paying IDB/VDB? There have suspicions that some airlines are doing this and it seems that the DOT would be interested if this played a role in what happened.

Note that I'm not saying that this was the case, only raising it as a possibility to be checked by the OP.

Unfortunately airline agents are often clueless about visa rules and do not know how to use TIMATIC correctly. Alternatively, they can be so sure that what they believe about visa requirements is correct that they refuse to listen or to check an authoritative source. (I had this problem with four DL agents in the SkyPriority check in area, a supervisor and a redcoat at NRT regarding Chinese TWOV rules. I was right but they almost didn't let me board.)