If the pilot was released and chose to fly back into domicile then it was perfectly legal. IF he was not released then it was not legal because of the 16 hour duty day. Back when I was a crew scheduler for a 121 carrier the only time we could go over a 16 hour duty day was if it was the last leg of a sequence and it was operated part 91. even then the pilot contract stated the crew had to accept the flight.
16 hour duty day is a tricky reg. Lets say a crew was at 14:50 minute duty day and scheduled flight time was 1 hour. the crew would have to push and be wheels up by 15 hour duty other wise if they got delayed while taxing more than 10 minutes then technically they would have to go back to the gate.
It sucks that mesa chief pilot isn't protecting his own.
Also remember there are 121 supplemental- domestic-and flag duty regs- I am assuming here we are talking domestic regs.