On 14 Oct 2016, at 19:54, Timothy Collinson <xxxxxx@port.ac.uk> wrote:

> Or run it as described but have the PC that is now an NPC (because the
> player got a new job and can't come anymore) locked up so that in fact
> no rescue is required?  She's just written out quite neatly.

Sure, lock up the NPC, but not to write her out neatly.
Instead the players still get to decide on the rescue and if they attempt it, it needn't be successful and might involve injury, death or recapture of the NPC, or the NPC might stay in the crew but is a wanted fugitive and cannot be seen off ship.
Of course, if the players do not attempt a rescue, you get an NPC with a grudge against the party to turn up in the future, or relatives of same.

Phil Kitching