I was on flight 297 from IAH to LAS on Dec 16th and heard the same type of conversation.

FA with long blond hair mentions to other FA (looking at the empty seat 3F) do we have a boarding pass for the empty seat - I will bring up a FA to that seat. The other FA says yes and hands it to her, but then a male FA is brought up and takes the seat. I am not sure if there was anyone on the standby list - but I am pretty sure there was - but nevertheless seemed out of place for the FA to suggest this without checking boarding passes in hand first.