A quick illustration is that, recently, facing a real possibility of having to exercise discretion in order to complete a duty, a junior CA said to me that there was "no way" that he was going to exercise "his" discretion. Funny, thought it was mine ! Bought him a coffee & had a gentle discussion about who is, actually, finally, charged with responsibility & why "my" manual refers to "Captain's discretion".
If that crew member feels unfit to fly the last sector you can use as much discretion as you like that they are not going anywhere.