It all comes down to one simple thing (in my simple mind) to determine who is at the majority of fault here:
How many boarding passes were scanned? My supposition is that it was only 3 and as others have speculated when 18 yo Mason was a no show his seat went to a standby pax. If, for some reason, they did scan 4 BP's with some sort of agreement of the GA, this situation should have never come up.