[erkyl]

I thought there was one extra seat next to the teen. That was the seat they sold to the standby not the seat the teen had purchased. That is the seat in dispute. Isn't it possible that the teen wasn't necessarily late, but it may have been assumed the teen was going to fit when she boarded, so they sold the ticket for the seat next to the teen to the standby. Meanwhile, on the plane, the teen didn't fit, the standby guest gets on the plane. The teen had to buy the next seat or get off, called a parent, got the ticket paid for. The standby woman had to leave because the teen was on the plane with a reservation, boarded and because of Southwest's policy had to buy an extra seat.

[Responsibility2nd]

Anything in your scenario is possible.

Except one thing. As you know by know (see links upthread) the teen was late.

Fat/skinny/whatever. If you’re late - if you miss the boarding - then you should be considered a no-show.