Andy’s suggestions are exactly what I would do and I can give at least one example where they worked:
A few years ago, I had a similar situation with a Flash designer who decided that he wanted me to double his pay halfway through the project. He was the best Flash designer in town and much of his work was intricate and undocumented. I couldn’t afford the new rate and he walked away.
So, I called an emergency team meeting and explained the situation. Back before we knew what to call it, we crowdsourced a solution by having some team members analyze his code while other team members spent the weekend cramming as much Flash knowledge as they could into their heads. We met again on Monday and were able to complete the remainder of the Flash designer’s work by that Wednesday. The completed work was actually better than what was specified and the customer was very pleased.
Moral: A good project team is more than the sum of its members.