My mother who is 84 and lives in a small village in Provence has just joined us for the Christmas festivities. Now she wants to go back home on the ferry and has just realised that her passport expired on the 27th December 2011.
Any suggestions on what we can do?

I'd start calling around, starting with the French consulate. Hopefully they will quickly arrange some sort of long distance approval even if just for the trip back.

They may of course just let her go back home, leaving again may be the issue until it is renewed.

How far is the ferry to just go and see?