If they are with Brittany Ferries it is worth them contacting their customer services if they are going to miss the ferry and they will probably let them go on the next one. My brother has been in a similar situation in the past and they let him use his ticket on the next one.My sis and B-i-L are stuck in France hoping they can get together enough kangaroo juice to get them back to the ferry at Caen. Every service station is rationing the stuff.
If they miss the ferry, they'll need to stump up for another ticquet.