This sort of a forwarded post does little to help anyone. The OP wasn't present and just posts something for someone else. If it was important to that person, why don't they sign up, post the details and engage in a discussion? If it's not important, why bother posting? If it's untrue or selectively untrue, it does a disservice to others who think that it might happen to others.

Since none of us, including OP, were present, nobody can say with certainty that what the individual says happened did not happen. But, it is extraordinarily unlikely. On the other hand, we don't even know the details of when or where this happened, how long the individual was at the gate, what kind of ticket they were traveling on, and the [U]passenger capacity (# of seats)[U] of the aircraft.

DOT rules, which AA is pretty careful about following require (absent some exceptions which may or may not apply depending on the details which the individual didn't bother to supply) and presuming that the passenger was properly checked in on time and at the gate on time (on time as required by the COC, not by the passenger's decision of what "on time" means):

1. That when a flight is in an oversale (not just overbooked, but oversold) situation, that the carrier solicit volunteers before involuntarily denying boarding.

2. That if no volunteers come forward, AA may involuntarily deny boarding. In those cases, AA must deny boarding in an order which is set in advance and not determined at the time. AA's COC detail that process and it is generally based on check-in time although it also includes fare basis and status.

3. AA is required to pay compensation to any individual involuntarily denied boarding based on a preset schedule up to 400% of the fare capped at $1,350. That schedule depends on the length of the delay the passenger will encounter. If AA can get the passenger to his destination fast enough, there might be no compensation due.

4. AA is required to report all such denials to DOT.

If OP wants to do more than report a rant for someone else and actually provide useful information, there may be useful information here. Otherwise, it is the standard answer: if AA was wrong, it deserves to cough up. If the passenger was wrong, SOL.