Not being a scheduler, I can't say what exactly they do. But from my observations, it seems like there are several possibilities to cover for a sick crewmember:
- leave the crew and plane where they are until the sick crewmember is well enough to fly.
- fly in a new partner for the non-sick crewmember and leave the sick pilot behind.
- if the crew was in place (or positioning) to fly a passenger leg, they can send in a recovery trip. That's one benefit to having a large fleet at the fractionals.
Like I said, I'm not a scheduler, so I don't know all of their tricks but those listed above do seem to be alternatives for covering for a sick pilot.