the rule is that a person of size may be required to purchase a second seat if the flight is full. Apparently, the first flight wasn't full, so it wasn't an issue.
thats they grey area I mean, make it a rule not a kinda, sorta, maybe rule KWIM?
I have seen stories were people have a round trip ticket, flew with family, flight was full, go to come home and that ticket agent says you need another seat? Said person was ok on the way to the destination with no problem but the way back there is one? major grey area
and like i said earlier, parents, guardian of the girl may not have known the seat rule~