You are making this up out of whole cloth. The Dad specifically said he bought an extra seat for the 18 year old on an earlier flight so he could have the baby fly in the car seat & be safer & sleep, as child did on the way out.
There was no refund and no game being played.
Dad asked when he bought the son’s ticket for the earlier flight and got the OK. These attendants knew nothing about that when they got themselves where they felt they couldn’t back down.
If that’s true the game is still afoot. You cannot simply use someelse’s tciket. I matter who paid or who’s was told. The ticket is for the named ticket holder only. Typically the older kid’s ticket would have been changed,, for a price, to the other flight and then the Dad woukd have bought the baby his own ticket for that flight. It seems they decide to play ticket roulette instead. My guess is that, if that Dad wasn’t just plain ignorant, he was hoping to avoid flying with the baby in his lap and didnt. want to pay the price for a last minute ticket.
Is there any confirmation that the Dad bought the older kid a new ticket?