How to handle a party member spontaneously not showing up?

I am the GM of a party that consists of two family fathers, one guy working on a oil platform and another freelancer working in the film industry. So it happens quite often that one member spontaneously cancels. If we are in a dungeon and it is all about fighting, this usually is not a big problem – I just tune the encounter down a bit – but concerning role-playing and storyline, I have a bit of a hassle.

Let me give you an example: The party travels to Cheliax. The elf becomes alcohol-addicted since he could not handle the death of another party member. The orc and the human decided to bring him back to a rehab center in Absalom. I already prepared the adventure, but the elf’s player gets a call from his boss and has to work spontaneously. So he cancels 2 hours before we start.

How can such a situation be handled?