Nothing new here. In the summer of 1966 all the U.S. Airline Pilots were on strike -- Except at American. As luck would have it, my mother chose American for our annual Visit - the - Relatives trip from SoCal to... Cleveland. I was a month shy of my 3rd birthday, so my mother bought me a child's fare. Everyone at American - LAX, from the gate agent, to the stewardess'es, to the purser, to possibly the Pope -- tried in vain to talk / buy / bribe my mother into giving up my seat to a full - fare passenger. No Deal : " I carried him for 9 months inside me, and 2 years outside -- he is NOT riding on my lap all the way to ... Cleveland. "
With the current state of affairs in the U.S. Airline Industry, today I would likely become a checked bag at the door of the aircraft, complete with a baggage tag on my ear and a fun ride down the slide !!!
Also back in 1966, when Ford was racing at LeMans, they had an issue with the windshields cracking in their GT-40 race cars. A few phone calls to Corning Glass, and new windshields were made and flown -- FIRST CLASS -- on TWA from JFK to Orly the day before the start of the famed 24 hour road race.
If the musician bought an extra full - fare ticket, she can do whatever she wants with the extra seat, in accordance with U.S. and International Flying Regulations.