The SO is not required to be a crew member on the recovery flight so technically ADFUS you are correct , but if the FO says sorry I can’t do it what is the Capt going to do order him to fly . Any time I have ever used discretion in the years before CC I always asked every crew member how they felt . If anyone said that they were fatigued that was it ,we went to the hotel no questions asked ,and as far as the company was concerned it was always my decision should they ever ask which they never did .